Not familiar enough with PieFed to give an opinion, but among Lemmy and Mbin, of things I can observe:

• Lemmy has far more visual candies / visual noise than Mbin, whose UI rather barebones
• But as Mbin has a more basic UI, it tends to break less and be more compatible with user scripts and filters
• On RSS, from my experience, Mbin links to posts properly through RSS, while, maybe it's version-dependent, Lemmy sites seem to have a bit of trouble with linking posts with links attached to their titles, usually opening the title's attached link instead
• However, Mbin doesn't seem to be able to fetch the post's body through RSS
• On newer versions of the Lemmy engine, you can block instances and hide posts, but not block domains linked in posts
• On Mbin, afaik, you can't block instances nor hide posts (both requiring browser modifications from my tests), but you can block domains
• On Lemmy, also maybe version-dependent, but it seems that instances don't host RSS for federated communities, while Mbin does (good for redundancy, I think)
• For microblogging, RSS doesn't work on Mbin (might in the future?) despite other microblogging alternatives having them, and integration of microblogging to Lemmy only happens indirectly
• On Lemmy, some communities seem to have an extra step to subscribing where you need approval after applying, while Mbin doesn't
• Specific to Mbin, but the error 404 issue from Kbin when blocking or subscribing to an user or community seems to be extremely rare with its successor
• Lemmy allows visualizing how formatting will look like before posting, while Mbin doesn't