Afaik from some folks who claim to have dug through the protocol: the protocol is needlessly complex with some very important parts of the network very unlikely to be decentralized (I think that was called relays) as it requires massive computing ressources.

The benefits of ActivityPub is that it is really really simple (basically just standardized json). Yes it has scaling issues, but those are solvable and partly just a sideeffect of actually being federated