The downside of message busses is that they can be really hard to debug for third parties. I run into this in SystemD, where a process will hang at startup waiting for a particular signal, and I have to hunt down all of the locations that signal could have been generated from to see which one failed. Much much harder than the old system where you would find the line you stalled on and then read up a few lines to see what it was trying to do.
It also becomes difficult to reason about what code will actually run when a message goes out. Top level handlers are ok, but they can spawn messages which spawn messages which... lead to me running out of mental capacity.
