I actually started on Kbin.social, but then it got shut down, Kbin died and now fedia.io seems to be the largest one running MBin. I like the interface on MBin and I guess it's good to have a diverse fediverse with different services, but at the same time, why use mbin when everyone congregates on lemmy instances? The local magazines on fedia are for the most part, quite dead, when compared to lemmy collections. In the end I feel like there aren't enough people to go around to support many more services like MBin and Piefed.
Better UI & UX.
As long as federation works it does not matter whether they are Lemmy communities or mbin magazines. So, why use Lemmy when you can make use of the same communities on mbin?
At least earlier there have been quite a few problems with federation not working between KBin and Lemmy, posts and comments not syncing. It does seem better now but at the same time it's hard to know if you're missing a few comments or threads or whatnot.
Kbin’s issues stemmed from lack of upkeep. Ernest is a good dude who did his best but he was truly a one-person operation and it showed over time. Mbin doesn’t have those issues.
A lot of the major federation issues in kbin were already fixed on mbin. Also, there were at least two major federation issues caused by Lemmy updates, which had nothing to do with kbin nor mbin and affected Lemmy instances accordingly as well. Of course, new issues can arise, but that also goes for Lemmy, or even just ActivityPub in general. But overall federation seems to be working fairly well right now.
I personally don't use mobile sites, but the only plus for Lemmy seems to be the thumbnail size. Aside from that they seem both kinda meh. There's plenty of apps for Lemmy though. Mbin only has Interstellar right now, but the Eternity devs said they at least plan to add mbin support in the future, which is what I used on Reddit back then too (Infinity).