[Brightcolours]

Is it really important? I mean: let's suppose the problem is field curvature & focus point. From an optic point of view, I agree that it's more precise to say "the lens suffers from field curvature" rather than "it's not sharp at borders". But the, on the field, if I want to take a photo with sharp details from center to border, in any case I'll see a problem. In the end, the lens would be not that sharp, independently on which is the exact cause. Right?

Wrong. With field curvature the corners will be sharp, unless you are photographing a flat surface covering the whole image frame. The lens will not be used for that too often, is my guess. With just plain not so sharp corners, the corners will never be very sharp.