Lemmy vs Piefed doesn't really make a difference, because it's the same type of Reddit-style platform (also called "threadiverse"), while Mastodon is Twitter-style. The interoperability isn't great IMO, because the usage patterns are quite different. e.g. if you reply to a Mastodon post on a threadiverse platform, the OOP on Mastodon won't see any of the replies, and are probably not even aware that there are threadiverse threads about their post. IIRC you need to @ the OOP in each of your replies, which you need to do manually in the threadiverse, which isn't particularly obvious if you never use Mastodon yourself.