Where did you read that it was "Bad engine, avoid", or "Unfinhed engine"?
Here, check this out http://devmaster.net...ty#user-reviews
To comment on the first review:
it is reviewing the now fairly old 3.2 version, most of the complaints raised are no longer valid. (Unity 4 (pro) has both a forward and deferred renderer and built in pathfinding) and weren't even valid for the pro version of 3.x. No built in AI editor, wtf ? You have to write your own AI code in every single general purpose game engine (that i know of atleast) on the market. I have a hard time even seeing how a "AI Editor" should work to be useful beyond a very specific game genre.
the second one is primarily bitching about the free version lacking features. (The free version is not, as the second review implies intended for independent developers, the pro version is, the free version is there for hobbyists or as an evaluation version).