I think the nature of our grassroots ecosystem, where we are still pioneering at protocol level, trying to fill gaps and overcome protocol decay (cope with the 'reality on-the-wire') has led to a bit of an upside down approach to software development. Where first technical facilities are built, then the impact of federation assessed in production, and then trying to make it address people's needs in subsequent improvements.
What does it mean if someone says they "joined the fediverse" with their app? By itself this says no more than that some form of technical implementation of ActivityPub is present in the project. It is a pure technical observation.
Looking at needs. What is the need for SocialHub? Maybe something like..
Support the communication and cocreation of all participants in the ActivityPub ecosystem to help foster healthy growth and evolution of the Fediverse.
There is no Forum in there, there's no dependency on apps. Apps are only the implementation of the need. Including the federation support in those apps. Currently the fediverse is highly app-focused:
"I have an existing app, and I add ActivityPub support so my app can federate with other apps."
And we currently have one reasonably mature way to do that, which arose from federated microblogging, but became a sort of baseline interoperability level (unfortunately).
@strypey raises an interesting question. What is a Forum if it becomes fully federated? Is it still useful to think in apps, or better to think in services? A Forum server delivering Conversation Services perhaps.
For SocialHub given the need above it made perfect sense to make it part of the Fediverse, give inclusive access to all fedizens. But the mere technical connection into a microblogging communications network did not take the need fully in consideration. Federate, deal with consequences afterwards. That's how we do it now.
Downsides to the technical integration, and where improvements are needed, are context collapse, and a shift of dev community communications to microblogging style, which is more ephemeral and fragmented by nature. And there is cultural impact. Is there more or less "sense of community" now, for instance? A more elaborate assessment on the extent to which federating SocialHub helped or hurt might be interesting.
Even more interesting I think, and where @strypey indicated a use case, is to venture beyond all the technical considerations that dominate the discussions. What do we hold conceptually in our hands? Envision the paradigm shift the fediverse brings if we wielded the technology to its full potential.