I just went through the simple, smooth process of creating a Discourse user ID based on following a link from MusicBrainz.org. It proposed a Discourse user ID "Jim_DeLaHunt" (with underscore) based on my MusicBrainz user ID "Jim DeLaHunt" (with space). It also prompted something like, "short, no spaces". So I changed my discourse user ID to my typical short ID: "JDLH".
Now I read that it's an advantage to have the exact same user ID on Discourse and MusicBrainz. But apparently the name rules are different on the two systems, in a way which ruled out keeping my Musicbrainz user ID. And the prompt for the Discourse account creation didn't suggest keeping the two names the same. So I ended up with something different.
I can imagine that unifying the user ID rules is one of the rough edges to smooth out, and I can wait for that. But I will need help changing my user ID on one or both systems in the future.
And, if keeping a unified user ID is really a benefit, it might be nice to find a way to suggest that on the Discourse registration dialogue. Since that dialogue is so admirably concise, I realise it might be hard to fit in such a suggestion.
I hope this feedback is helpful in smoothing out rough edges.