Of course, nothing is perfect. There are bugs and performance issues, but these also exist in Unity so that's nothing new to you. What IS new is that Godot gives you the source code, so that you can debug and fix issues yourself. You're not dependent on your vendor's priorities!
Performance: Godot is optimised for general-purpose cases, and sometimes games hit the specific cases it's not well-optimised for. This is true for any engine; there is always a trade off. It was useful that we could optimise GridMap for our specific open world use-cases though!