As mentioned (hah!), they are supported in comments on PieFed (they’re clickable to resolve to the profile, you’ll be notified if someone Mentions you, and you can use them to send your comment beyond where it would normally go).

The limitations are largely due to inter-op with Lemmy - it doesn’t support Mentions in post bodies (in either direction), so that was one reason why I didn’t add support for it (the other reason was I wasn’t sure if it should be regexed like comments, or if the plan was for there to be a separate field, like hashtags).

The other Lemmy inter-op thing is that both sides will reject Mentions if they don’t already have the relevant Community (and Lemmy will reject them if they have the community, but it doesn’t have any local subscribers).