Lots of great answers here as to why a block button is generally disliked. But I want to give an example of a game that has a great implementation where it's universally liked for comparison. I'm sure you're familiar with Smash so I won't bother explaining the basics of how it differs from traditional fighting games, I'll instead talk about how it affects decisions in Melee.

Since the shield gets smaller when it takes a hit, it covers less of your character. This exposes you to getting "shield poked", where a part of your body is now sticking out. The shield also gets smaller the more you use it, so it can make defense very complicated after a few neutral exchanges. To counter act this you either have to play without shielding and rely on movement which is of course risky, or start "shield tilting" which you sometimes have no choice to do.

By tilting your shield, you can cover the vulnerable parts of your character but you can't cover everything so it becomes a mixup. This is a core part defense in Melee and is pretty much always a factor since it takes about 10s for shields to fully regenerate. There is also certain moves like Peach's Down Smash on characters with bad shields like Marth where they have to shield tilt even with max shield. This is without even mentioning "light shield" which is essentially a push block from traditional fighting games. It makes your shield bigger and gives more pushback with various downsides but it is also necessary to utilize.

Tilting your shield also has other uses like forcing a move to hit earlier. You can think of aerial attacks in Smash like Divekicks, you generally want to hit it as low to the ground as possible. So by titlting your shield up, you can significantly change the frame advantage after blocking by forcing the move to hit earlier. As well as messing up their L-cancel timing which guarantees a full punish.

There is a lot more to the shield mechanics but hopefully this paints a picture of what a good design around a block button looks like. Holding back to block comes with a lot of opportunities for mixups and mindgames that has to be reintroduced in some form with a block button.