Characterising any of the Unity, Unreal or Cryengine trio as a 3D engine isn't quite right, they are full on game systems with physics etc. and especially in the case of Unity complete software ecosystems in their own right.

The further you get from utilising the core functionality of each engine the harder of a time you're going to have, and this constrains the genres you can work in. This is the main reason that a lot of 2D titles developed in Unity purely for iOS <-> Android portability have performance problems (things like the alpha cutout stuff goes a long way to helping here), while their showpiece games, such as those by Madfinger, that simply use the built in physics run perfectly. Sadly I can't be specific, but I have run into some amazingly simple games that ran like absolute dogs thanks to their inappropriate choice of middleware.