For what it's worth, I've only watched in order to try and understand other people's viewpoints.
More specifically, I think he's terrible because he has mastered the ability to tease out the base instincts of people with his messaging, which makes it hard to either agree or disagree with his statements with logic. He can point to some kernels of truth, and you are left with people saying things like "that's just dogwhistling" when attacking his viewpoints. In other words, he riles up, doesn't cause people to think critically, and overall lowers the level of discourse out there.
To me, he seems like a whiner who disingenuously argues against things in a way to bolster conservative talking points. But, I expect the majority of this is selection bias, and only the 'worst' clips are making it into my filter bubble.
Might just be me. I dislike 90% of fox news but I listen to Carlson sometimes and never find him to be horrible or BS-y (admittedly I don't listen in all the time so I may be missing some stuff)