ActivityPub does have a big advantage over the other options - it's proven to work, at scale, for a variety of systems. It looks bad compared to some other options partly because it's had much more battletesting to find those weaknesses.
Identities independent of account is an important concept, spearheaded by the At Protocol but their design is quite complicated and requires users to own a domain and I'm not convinced (yet) that it'll work in practice.
Mastodon would need to add support for the necessary object types and protocol extensions. Things like comments on issues or merge requests may federate without special care, but the experience will be pretty bad out of the box unless Mastodon chooses to implement the extensions necessary for project tracking.
Mastodon does nothing with many types of ActivityPub and ActivityStreams objects because they don't make sense for what Mastodon is. When you send a Create status for a Polygon or a Service object to Mastodon, there's very little Mastodon can do with all the data you provide it, because that type of object doesn't really make sense. There are uncommon types that could be rendered in a timeline, I suppose, but I hardly expect to see Mastodon render "Sally indicates that Mark is her brother-in-law".
Mastodon is just one application using ActivityPub, there is no imperative for other AP applications to specifically be compatible with it (although many are, whether on purpose or as a side effect).
Forgefed creates an extension to the ActivityStreams format. I don't know how Mastodon, Lemmy, KBin and others will handle those. However, they will be able to handle the vanilla parts. So perhaps, they can participate in notifications and discussions/reviews.
ForgeFed is an extension to ActivityPub protocol that allows bridging forges wrt issue tracking and merge requests, but innovating further on top of the protocol. For instance by adding support for Object Capabilities [0], inspired by Cap'n Proto and the Spritely Institute.
The project would be greatly helped by having more contributors put their eyes over the specs and adding support. Gitlab is working on ActivityPub-based federation [1] and will hopefully become a great reference implementation.
I was personally hoping to see an implementation of this concept on top of Matrix (in fact, I know someone who claims they are going to someday build a git hosting backend on top of Matrix).
I'm bummed they chose ActivityPub for it, as it's not a great system. That aside, I'm just happy it exists.