@Ikea expert
It's interesting what you wrote about the mounting rail. That was exactly my line of thought, that the rail can be helpful with a crooked wall. With straight walls, I also have no problems attaching the cabinets directly to the wall.
As mentioned before, I have no experience with assembling a Metod kitchen and was a bit uncertain the last two days regarding the mounting rail. But I think I will try it with the rail. In the worst case, if I can't handle it at all, I will attach the cabinets as with Faktum.
I am really curious how the method with the mounting rail will turn out. My walls are not necessarily how one would ideally wish them to be. I would not have wanted to do my first kitchen assembly with these walls.