ORMs are a pain to use. In addition to know the domain you need to map from and the domain you map to, you now also have to understand the mapping process.
...in exchange for dramatically pared-down and simplified code, consistent data access practices, and hundreds of hours of developer time saved. Driving a car is tough too - how to steer, drivers license, gas, insurance, what a PITA. Yet somehow it remains preferable to walking in many cases, despite the latter being mastered by most two year olds.
The same should be said for ODMs as well. A document might be a little more straightforward to map to an object but there is still plenty of miss-match.
I'll agree with this. Document stores don't solve the object-relational impedance mismatch, but they do help (and personally, I find they help more than "a little").