Isn't it impossible to optimistically update the UI with this model? You need to wait for a server roundtrip before you can display the results. There are certainly cases where that's OK, but it can make an app feel a lot less snappy.
Optimistic updating needs a serious amount of code and robust error handling on the client and is therefore non-trivial.
This approach seems aimed to reduce the amount of work while being relatively fast.
Unless you abstract away the non-trivial part with something like Firebase :-D