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.