It's not any more complicated to develop for than an equivalent 3d-space game, but almost none of the tropes and techniques that work for a flat experience like playing Counter Strike translate. People are figuring out basics like how to make picking something up feel good/right, really basic gestures and actions take on a lot more importance when you're supposed to feel them as a natural part of yourself. It used to be "press A to open door", now you actually have to open the door, and getting that right takes a lot of trial and error.