The first thing is it needs some testing and bug fixes, statistically it is essentially unused right now.
Neither me or @AndreK can even start on adding it to the phone apps until the code is tested and works, there is documentation and examples and it has been integrated in a mostly transparent to the user way into the text message app packet type on the device. The overwhelming majority of users won’t use the feature and should not have to deal with it at all.
Some of this is code, but a lot of it is going to be coordination with the devs working on assorted clients. There are also a lot of messaging features that are coming first and I think many of the theoretical use cases for for multi channel are handled by features we have that are not implemented across clients like DM’s, replies, tapbacks etc.