First off, let's frame this question in the context of a good REST API and an existing rich object model on the server-side, as well as modern web architecture (MVC frameworks like angularjs).
I'm debating whether or not to use a rich object model with my next project. The alternative is just using very light-weight POJO objects (pretty much whatever the API gives me) and using these in a non-persistent (between views) manner, i.e. no "application state". Just load what you need for the current operations and then get rid of it when you don't.
The risks of a rich object model that are giving me pause are:
1. Testability - it could be harder to unit test a complex object model
2. Performance - with a large object graph, the performance of the app could suffer. Contrast that with lightweight POJO's that get created and destroyed as needed, the app stays nimble.
3. Complexity - a complex object model could be orders of magnitude more complex than a lightweight app that just loads what it needs at the moment. This could also make it harder to modify...
4. Harder to modify - a rich object model creates a multitude of dependencies. This makes it harder to change things down the road.
I'm leaning towards a lean, light-weight web app that just loads what it needs at the moment and destroys what it doesn't.