It's mostly opinionated. systemd is written in C, uses a consistent config, is documented well, has a lot of good developers behind it, is very fast and light, and does what it's doing very well. Since systemd also is split up into multiple parts, it still follows the "do one thing, do it right" philosophy.
There are some people that believe that systemd "took over" the init systems and configuration demons of their distro, and does "too much." It really does quite a lot: it can replace GRUB (by choice), handle networking config, all the init stuff of course, and much more.
However, I have lived through the fragmented and one-off scripts that glued distros together. Some distros used completely custom scripts for init and networking, so you had to learn "the distro" instead of "learn Linux." They were often slower, had worse error handling, had their own bugs, were written in various scripting languages like tcl, Perl, Bash, POSIX shell, etc. It was a mess.
The somewhat common agreed-upon init system was System V, which is ancient. It used runlevels, nested configuration (remember /etc/rc.d?), and generally, it was mostly used because it was battle tested and did the job. However, it is arguably esoteric by modern standards, and the init philosophy was revised to more modern needs with systemd.
You can probably tell my bias, here. If you have to ask, then you probably don't have a "stance" on systemd, and in my opinion, I would stick with systemd. There were dozens of custom scripts running everywhere and constantly changing, and systemd is such an excellent purpose-built replacement. There's a reason why a lot of distros switched to it!
If you want to experience what other init systems were like, I encourage you to experiment with distros like the one you mentioned. You might even play with virtual machines of old Linux versions to see how we did things a while back. Of course, you probably wouldn't want to run an old version of Linux for daily use.
It should also be mentioned that init systems are fairly integral to distros. For example, if you install Apache httpd, you might get a few systemd .service files. Most distros won't include init files for various init systems. You can write them yourself, but that's quite a lot of work, and lots of packages need specific options when starting them as a service. For this reason, if you decide you want to use a different init system, a distro like the one you mentioned would be the best route.
Back when systemd was a hot topic I jumped on the bandwagon of using systemd-less distros just because people were telling me how bad it was. To this day I still use openrc but the reality is that systemd works very well and is easy to understand and use. The average user gains no benefit to using another init besides having a better understanding of how the system works.
Short version: some people (I'm one of them) object to systemd on grounds that are 75% philosophical and 25% the kind of tech detail that's more of a matter of taste than anything else. The older sysV init is a smaller program, which means that it has a smaller absolute number of bugs than systemd but also does less on its own. Some of us regard "does less" as a feature rather than a bug.
If systemd works for you and you don't know or care about the philosophical side of the argument, there is probably no benefit for you in switching.
Systemd is huge. It's a complex project that covers not just the init system, but also process management, networking, mounts, sessions, many other things. Many people think its monolithic design run counter with the Unix philosophy and wish to use distros without systemd.
5 minutes of fame when Debian said "if you want other init systems, maintain them", soon-to-be-Devan folks slammed the door and effectively ruined the chance of multi-init debian by fracturing efforts into their fork instead. But hey, all the news were abuzz about them.
If you're a new user you'd be better off moving on from here and not paying much attention. It's a hot topic full of opinions that everyone will want to force on you.
If you really want to swap out the init system there are some things you need to know.
First, do you need a desktop environment(DE)/window manager(WM)? If so you'll need to find a DE/WM that is not going to demand you use the mainstream init choice which currently is SystemD.
If you want to use Gnome from your chosen distro repo's then chances are it will pull SystemD with it.
If you want Gnome but not SystemD you're gonna be building that beast from source every update and for the most part you'll need to go direct to Gnome for any issue/bug you fall over and this too will be painful.
Simpler WMs will be more forgiving and will only rely on either xorg or wayland and will happily run on any init.
There will be other packages out there that also demand you use SystemD, so you'll have to find them and decide if you need them or if there are alternatives that don't have a hard dependency on SystemD.
All the current usable inits are written in C or C+ (except for GNU Shepherd, this is written in guile).
The benefit of swapping out the init system is mainly down to choice, necessity but again this all boils down to what the installation is for and what will it be doing.
All of the init's (except for epoch) provide parallel service startup so if boot time is a focus test each to find the fastest for your platform, Not all of them provide per-service config.
For example one can cobble together:
minirc, busybox, syslogng, crond, iptables, lighttpd.
And the end result would be a relatively secure webserver with a small footprint, you could further extend this with nginx to sit in front of lighttpd to provide waf and cache features.
The biggest bug bear with SystemD is that it writes to binary log files and even though it can be configured to generate plain text, if it falls over in a bad way you will still only get a binary log file and if you're in a situation where your only access is say busybox for emergencies. In this instance your only option is to boot from another systemd distro and mount the broken install and run:
Other than that many take issue with SystemD trying replace parts of the system that many say don't really need replacing like sudo, fstab, resolv.conf, etc but again these statements get full of opinion and don't help us truly way up the differences and some of the SystemD alternatives misbehave or become hard dependencies other projects which makes it harder to disable parts and swap out to your chosen package.
I've tried to be more objective with this response and keep as much of my personal opinion out of this, But here is mine:
I don't really like it but to make it easier to get support for my OS I put up with it, I daily drive arch and so must accept it. I could rip it out or run artix, I've gone down this path and got fed up with jumping hurdles to get what I wanted so went back to Arch and now I disable parts of it I don't need/want, have it generate text log files, use openresolv and other choices.
Devuan is the outlet of a bunch of people that don't want Linux to evolve, become better and have more flexibility because it violates the UNIX philosophy and/or it is backed by big corp. Systemd was made to tackle a bunch of issues with poorly integrated tools and old architectures that aren't as good as they used to be. If you look at other operating systems, even Apple has a better service manager (launchd).
Systemd is incredibly versatile and most people are unaware of its full potential. Apart from the obvious - start services - it can also run most of a base system with features such as networking (IPv4+IPV6, PBR), NTP, Timers (cron replacement), secure DNS resolutions, isolate processes, setup basic firewalls, port forwarding, centralize logging (in an easy way to query and read), monitor and restart services, detect hardware changes and react to them, mount filesystems, listen for connections in sockets and launch programs to handle incoming data, become your bootloader and... even run full fledged containers both privileged and non-privileged containers. Read this for more details: https://tadeubento.com/2023/systemd-hidden-gems-for-a-better-linux/
The question isn't "what is the benefit of removing this init system", it is "what I'll be missing if I remove it". Although it is possible to do all the above without Systemd, you'll end up with a lot of small integration pains and dozens of processes and different tools all wasting resources.
It may speed up your boot time, at least it happened to me on Void (maybe the reason is how minimal this distro is though). I personally prefer runit over systemd in how it handles services, but honestly you most probably won't notice a much difference - definetely not worth reinstalling whole system.
You should embrace systemd. It's actually good. Replaces all startup scripts, logs to a common log, even has scheduled systemd jobs just like cron but better, since they can have proper dependencies. Want to run something right after network stack is up and working? Easy with systemd, more difficult with cron and more hacky.
I don't know anything about Devuan, but the init system is replaced, not removed. You need an init system. Devuan probably has something more barebone than systemd.
I don't personally like SystemD, but Devuan sucks. They advertise "init freedom", but in reality all of the scripts by default are just sysV init scripts that runit and openrc can't control.
For a desktop user I don't see any significant benefits to replace systemd.
But also no-systemd distros works fine. I was impressed during my try on Alpine Linux, that uses openrc instead. The text printing during OS startup is so short that the terminal didn't scroll. The bluetooth worked flawlessly. But it is a small community distro, and Alpine is limited by other things than the init system.
The init system is a problem for people that have to deal with services.
On political aspects, IMO FOSS works easier with small and focused components that can survive with spare time developers. I can't make critisicms on technical aspect, I'm not a good programmer, I just notice systemd seems to works fine.
Red hat has man-power and capable of large contributions to Linux distros so they leads the innovations. All big distros switched to systemd, now its hard to avoid.
I would like to support smaller FOSS-friendly systems but I use Arch because I need recent versions and the anti-systemd arch-forks are harder to use. I'm a weak guy.
In short, as an user you should be fine by keeping normal Debian. If for political reasons you want a no systemd distro, the easiest is to use MX Linux with the default init.
SystemD replaced a variety of Linux init systems across different distros almost 10 years ago now but it is still resented by a significant and vocal section of the Linux community.
SysV init is crap, but so is systemd as init process. One example is that an admin needs to know why the system does not boot properly. In this case the kernel messages help. systemd is not helping here.
I've currently one problem that I need to solve, but I need 2 people, one to make a video, the other to press Ctrl+Alt+Del to capture an error message that appears for 0,1s after sending the key sequence, when my PC does not boot. This is crap! Why the hell it does not boot occasionally, I have no idea and I've been an Linux/Unix admin for 25 years now. Why I cannot find it? Of course because systemd doesn't even log it!
This is brand new when systemd appeared. I loved to see the kernel messages to full extent...
My problem with systemd is that since I'm practically forced to use it that it's flakey in starting services after boot (independent of service and distro). Since systemd I had to install monit to check if all services came up. Didn't had that problem before. Or I forgot, it's been a while....