Jerome Dalbert on his blog:
Splitting your Rails controllers when they have a very specific scope, too much logic, or too many mixed concerns can have a lot of good side effects in your code.
It doesn’t mean that you never abstract. It just comes later down the road. At some point some logic needs to be shared by several controllers. Sometimes even a splitted controller with only one public method gets too big. Et cetera. This is where concerns, model methods, possibly background jobs, and even sometimes service objects (hopefully not too many) come into play.
The more your app grows, the more time you will need to spend to understand it, no matter how clean the code is. But splitting your controllers makes things easier.