Well, for one thing it only works asymmetrically. It's fine if you have a very specific source of issues that you can isolate and cut off, but it's not really useful if what you have is hostile users across the network. And it only protects the larger space. For smaller instances it's a choice between functioning as social media or not existing at all.

It's extremely far from a magic bullet, it is not resilient to large scale, systemic issues and the only reason its limitations haven't been apparent is that the AP ecosystem is too small to suffer most of the issues of larger social media.

Aaaaand it's designed to function via the petty squabbles of FOSS developer arguments, which I hate anyway. But that's a me thing.