For example in the Lemmyverse you don’t follow people, but communities. From the Mastodon side, they’re visible as ordinary accounts. The first post in each new thread then shows up on your timeline; you can reply to it, and the reply will federate correctly. It’s not seamless, but it works well enough both ways. That’s great.
However if you try to follow someone’s Lemmy account from Mastodon… it will let you, but you’ll never see any of their posts, and there’s no indication why. That’s not so great.
People need to be able to trust a protocol. It’s the difference between an e-mail being returned as undelivered (preferably with a stated reason) and the same e-mail being silently dropped. Bad enough already that ActivityPub is more of a framework for creating related protocols that may or may not be interoperable, in part or at all. If developers do this on purpose, the protocol becomes a lie, and people start going where they can communicate predictably, even if much worse.