You saw the one that was a called strike right? And sure big leaguers don't swing at them. Right. he has 11 walks in as many innings in the big leagues. His WHIP is 2.25. You don't throw a slider for strikes, you throw it in the dirt when you already have 2 strikes on a hitter or are at least in a pitcher's count. Since he can't command any other pitch, when he faces a big league hitter he rarely gets in a pitcher's count and actual major leaguers know they can just spit on anything that looks like a slider. anyone with a good slider is going to have a nice highlight reel given enough time, but you gotta be able to command another pitcher for it to be of use.