The two interoperate perfectly. There are some Piefed only features - ~blaze_eu@piefed.social appears as a link to a multi-community (feed) on our end, but I don’t think that’s supported by Lemmy yet. The developers communicate so I think the plan is to eventually get that working in Lemmy as well.

I also reacted to your post with a smiley face, but on your end you’ll just see it as an upvote. There are some more technical issues, such as (if I remember correctly) Piefed supporting the moving of discussion between communities but Lemmy not yet having implemented support for it. Some Piefed communities support making posts in the Q&A format where OP can highlight the best answer, which will just render as a normal discussion with no highlighted answer in Lemmy. But stuff like that is really the extent to the incompatibility. There’s nothing that is really felt by users of either platform.