Making people guess ambiguous crossups is more lazy offense than anything.
I notice that a lot of people keep mentioning ambiguous crossups. I think people who don't play/enjoy these games don't realize that truly ambiguous crossups aren't happening nonstop. The player is almost always in control.
For example, with Sakura, I have a midscreen crossunder setup where I can always decide which side I land on, and the difference in timing is just delaying a part of the combo by a scant few frames. I also have a corner crossup mixup that depends on me hitting a button with a minute difference in timing to decide which side I hit on. While it's basically impossible for the defender to see, and they must make a read, I always know what I'm doing.
In my mind, a TRUE ambiguous crossup is one where even the attacking player has no idea what will happen. Those really dont happen all that often.
I personally prefer B2B because of the dynamic footsies options it allows, but obviously it would never work in a game as wild as MK! In MK, a block button feels delightfully natural as far as how it relates to the gameplay. I would love a 0 frame recovery when I release block though, even if there is a release animation (it should just be able to be canceled).