While that's largely true when dealing with pure business logic, there's a maddeningly large amount of UI logic, and hybrid business/UI logic that becomes really annoying (or outright impossible) to handle in pure MVVM with WPF. Reasons for this include that many user interface properties aren't exposed nicely to allow data-binding.

Probably the most infamous one is that the WPF listview with multiple select enabled doesn't allow you to bind to the collection of selected items. Instead you have to do all sorts of work arounds, that while individually aren't too bad, when put all together, makes all the other hard work you put into doing MVVM on the components that you fully control super frustrating.