ReVanced lets you sign in so you can do things like thumbs up/down videos (it also comes with “Return YouTube Dislike” so you can see downvote ratios), interact with your playlists like Watch Later, and comment. Piped, to my knowledge, does not.

I’m not sure where Piped stands on PIP and background playback. ReVanced gives you both of them, but if I had to guess I’d say it’s likely that Piped will support PIP but not background playback, since it seems like PIP is something Android lets you have nearly for free (in terms of developer effort) but background playback seems to need more work to support.