First of all, let me say that this article was a great, practical guide to using Redux.
MobX allows you to make “action” functions and, with the flip of a switch, raise an exception if you try to mutate any of your state outside of an action. Combined with the MobX devtools which log every state change that occurs and where it came from, it seems to me that you can get quite a bit of predictability from MobX.
I have used both Redux and MobX, but neither with giant state or reducer/action trees. I’m curious where you think MobX will fall over when you have lots of state.