I feel like it would be much better if I could use a single identity and a single "ActivityPub server" for all my applications.
In the current situation, being able to retweet my PeerTube video from my Mastodon account has little advantage about just tweeting a link to it. Likewise, commenting on a Lemmy thread from my Mastodon account is ok but I can't really see the thread as a thread without logging into Lemmy. Watching a PeerTube video, I can post a comment using my Mastodon account but that requires 5+ click and a change of application.
I am not sure why the ecosystem went this way. The client-to-server protocol was readily available when the Fediverse was kickstarted.