Looking at the specs, it’s a promising concept, but its considerably less mature than ActivityPub. It’s also struggling with some of the same challenges:
What is Missing?
Non-Public Data: mechanisms for personal and group data sharing are planned, and should support a wide variety of application functionality. These will be large additions to the protocol framework. We recommend against simply “bolting on” encryption or private content using the existing protocol primitives.
Protocol Governance and Formal Standards Process: once the basic protocol framework has been shown to work between multiple interoperating organizations, the plan is to bring the protocol specifications to an existing standards body (like the IETF) for revision and independent review.
The first point is one the Fediverse struggles with (like the fact DMs aren’t private). The second is where ATProto falls behind ActivityPub - right now Bluesky is the only significant org running the protocol, and as such they’re still establishing standards.