I think you need to be more careful with your use of “scaling up”. You mean vertical scaling, while ActivityPub is good in horizontal scaling.
Indeed, with ActivityPub you will probably never have a single point that has a global view of the network, which (at least for now) is the way ATProto presents the network. Arguably that is not a desirable feature anyways, but lets just say that people have different opinions on that.
I see no reason why they would be worse, other that the initial purchase price.