I use booster and it's cool. I don't see any noticeable difference in boot times but the image generation is much faster. mkinitcpio would take several seconds while booster takes about one.
First time I tried it it didn't boot because of something missing in the generated image. I tried a universal booster image (set universal: True in /etc/booster.yaml) and it worked. Technically this builds a larger image than necessary but it's still only 34MB and takes a second to build, so I never bothered to troubleshoot what was missing. The universal image even handles luks encrypted root partitions without additional configuration of booster (you still have to configure kernel parameters).
Another issue I noticed is that if you use grub-mkconfig and your only initramfs is booster, it will generate an incorrect main boot entry. It will add booster as an option in "advanced options" so your system is still bootable if this happens to you. The quick fix is to manually add the initrd entry under the main menuentry in grub.cfg.
boot time difference feels like in the realm of margin of error
the biggest difference however is that booster builds the initramfs much much faster while mkinitcpio slows down every kernel upgrade espcially on slower laptop cpus
I'm wondering how both mkinitcpio alternatives work on non systemd boxes with full disk encryption. With both, I refer to dracut and booster. On its origins I believe dracut was pretty tight involved with systemd, and booster is developed/maintained by an arch developer/user if not mistaken, and arch supposes systemd, though none of those things actually mean non systemd boxes are not supported.
I'm also wondering if the initrds generated can be launched by grub (I do /boot partition encryption/decryption with grub), and I also do / full partition encryption with luks. This booster issue sort of indicates as of now booster initrd images can't be loaded by grub...
Grub can load booster images, the issue is about incorrect grub.cfg generation.
What they're saying in the issue is that grub-mkconfig will not create a correct "Arch Linux" menu entry for booster, but if you go to "Advanced options" and choose the "booster" menu entry it works. I can confirm this. It happened on the system I'm currently using.
Specifically, the problem is that grub-mkconfig does not add the booster image to the initrd of the default menu entry. You can add it manually. For example I had to change this
initrd /intel-ucode.img
to this
initrd /intel-ucode.img /booster-linux-zen.img
If I recall correctly this issue was not present last time I set up a system with booster. It might be a regression or maybe it only happens in specific system configurations.