In addition to what the other comments mentioned, PieFed has a number of features that Lemmy lacks - like Categories of Communities, hashtags, the initial wizard to sign you up to communities that you are interested in, showing the community side-bar text alongside every post in it, unique messages offered irt particular instances or news sources, etc.
It will take some time to get an existing app to catch up to all of those, when Lemmy lacks those features entirely.
PieFed is also extremely much more efficient than Lemmy if you are using a data plan: iirc it uses ~25x less data per post. Tbh any server - including PieFed.social - can be slow or unreachable sometimes, and while PieFed.social tends to rarely ever be slow, it does suffer from more reliability issues (there are several other instances running PieFed though, if someone doesn't want to be on the flagship running beta software that's constantly being upgraded - though that has benefits as well as disadvantages, as roughly once a week if not more often you'll discover entire new features have been added:-P). Anyway, I'm loving PieFed over Lemmy!