If you thought goto was bad, observers are basically a comefrom statement. You change something and literally anything can happen. Once code starts getting large, these will start causing chain reactions and the code gets hard to reason about.
The goal is for nearly all of your observers to be nothing but a cache-clear on a pure function ("computed", in MobX's case). Chained reactions (reactions that trigger other reactions) are very rarely necessary and should be kept to a small enough number that you can model the entire state-graph in your head.
