Unity Has number of advantages, Some of them are here:-
Supports Multi Platform
Unity supports development of games in Multi Platform (iOS, Android, Windows, Web, PS3, and Xbox) without separate code. Unity allows creating 2D as well as 3D games. Unity allows Multiplayer games.
Unity has quite comprehensive library in terms of components like Rendering, Physics, Sounds. Physics can be applied without much complex mathematical calculations. It supports Reflection, Parallax and Bump Mapping.
Unity provides optimization of games through Profiler (Profiler tracks the performance), Batching.
Unity is flexible with multiple Plug-in such as NGUI, Orthello, Prime31, Facebook SDK for all the platforms.