I have never liked the licensing terms of Unity (the decision was made before Unity 5). Basically, I wanted to use C++ because strict memory and performance management is required by this game genre. Earlier it was developed in a C-like script language so porting was not too bad to UE4. Esenthel engine was also a possible choice, it uses C++ too, has nice rendering, simple, and fast, but its source license is a bit expensive comparing to UE4, beside having less features (Esenthel 1 was free, 2 requires a subscription). For me Cryengine’s main drawbacks were the asset pipeline (as a programmer I own no Maya or Photoshop) and Lua, what I felt a not very useful interim step.