This assumes you can just plug in the engine and all is well and good. Many of these engines are optimized for certain types of games and do not play well when

This assumes you can just plug in the engine and all is well and good. Many of these engines are optimized for certain types of games and do not play well when bent against their will. There is a large cost of learning to utilize the engine and get the best performance out of it. You are also heavily committing yourself to something where it is possible a wall could be hit and there would be no way out of it. Moving away from one of these engines is very expensive and may involve re-writing the majority of the game. It is not something you want to decide lightly.