Unity uses an IMGUI approach and it makes all the difference in the universe. Overriding an OnDrawGizmos method to quickly get at an editor viz of a new component is super efficient. There are some sharp edges like forgetting to set/reset colors, etc, but I much prefer these little annoyances for the convenience I get in return.

AFAIK, UE relies on a retained mode GUI, but I never got far enough into that version of Narnia to experience it first hand.