Soroush Khanlou: (emphasis mine)
The lack of concrete naming makes this class's responsiblities grow endlessly. What functions should go in a view model? Nobody knows! Just do whatever.
The most charitable thing that I can say about this pattern is that it changes your kitchen sink from a view controller, which is not an object that you own (because it's a subclass of an Apple class), to a view model, an object that you do own. The view controller is now free to focus on view-lifecycle events, and is simpler for it. Still, though, we have a kitchen sink. It's just been moved.
One of my current projects is a Knockout MVVM project and I agree with everything Soroush is saying. We've pretty much decided that we are just going to use good-old MVC for our project instead of MVVM because of it's needless complexity. I'm reminded of a Gary Berndhart talk, Boundaries in which he talks about the Ruby community's struggle with where to put their big balls of mud.