The one I use - memmy - frequently has issues with widgets that stop responding, and currently is glitching such that the upvote/downvote buttons are superimposed over the posts. Search results show all communities as having 3k subscribers even if there’s actually only single digits. If you highlight text to make a link, it overwrites the text with the empty link rather than making the text into a link. Mlem and Liftoff - the other two I checked - have their own issues.
I think we can also do a better job hiding the complexity of federations from novice users and cut down on the impact of bot-based crossposting by detecting that the lines articles are identical. I could see, for instance, discussions being merged on the client side.
I found reddit neither usable nor interesting before Alien Blue, and I suspect there are a number of potential users out there who would onboard or increase engagement here with a better UX.