ABS stats this spring show that only 52% of ABS challenges have been successful. 44% for hitters and 59% for catchers, who have a better view than umps. If fans didn't have the K- zone box on TV and tried to call a game we'd probably all be a lot worse.
They have a tough job like you said. Balls are moving through the zone too. That pitch looked like a ball, but as people have shown with ABS, any 12-6 breaking ball at the bottom of the zone will show up on K-zone very differently depending if you're measuring the K zone on the front of the plate or middle. They've shown how a curveball clipping the bottom-front of the zone can end up in the dirt.
And imagine having a job where hundreds of thousands of people, if not millions in some cases like this, are ready to jump down your throat when you make an error at work.
People seeking perfection will have their wish over the next several decades as most of our jobs can/will be replaced by machines. People will disagree and that's fine but I'm on team human.