This is not hyperbole. I truly believe that it is so much harder to play good golf without a flat left wrist (for right-handers).
You get your wrist cupped when you have a bad grip, a right-hand that’s too dominant, or by twisting the face open at take away.
A cupped wrist opens the face which costs distance, solid contact and directional control (slice usually). What happens is that we have to rely on compensations to square the face at impact and this is just too hard and not repeatable.
Check out this video as I explain more.