I don't know compared to Unreal but Unity has several hidden performance issues. I am currently working on Bullet Boy [1] and we had several obscure problems that you have to fix with a lot of trial and error. Specific combinations of components and attributes can cause frame rate to drop because of the internal organization of the engine (moving colliders without rigidbodies comes to mind).
That being said, it is really easy to use. The asset pipeline is great and artists learn to use it very fast. It works really well for small teams.
My recommendation is that you should consider it if you are not planning to make a very performance intensive game.