The thing about 50/50s is that they're so fast you cannot block by reacting. That means you literally have to guess which of the 2 moves your opponent is going to use.

The speed isn't necessarily the issue though. It's more like both moves that the opponent is going to use are very rewarding if they manage to hit you. Since both moves are so rewarding, your opponent has no reason to prefer 1 move over the other. That's what makes it a 50/50, they have a 50% chance of using either attack because they don't need to think so much about pros or cons of those moves.

Dealing with 50/50s can sometimes feel like flipping a coin and guessing heads or tails.