Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars.
The lighting system is bad. At least, now, they provide shadows and in their Adam demo, there are volumetrics.
Many parts are overtly complicated, but its easier than Unreal.
The probability of a memory overload when producing detailed levels is too high.