The old consensus used to be that they're "inconsistent."
I had one who had pretty much a lot of good ratings except for only three pitches. He won one Cy and had a runner-up as well. Actually didn't have a great start to his career but got much better after age 30. Great DUR/STA so he had a bunch of games where he'd give up two or three runs in the early innings then go lights out for six after that. I had to learn to trust him, but he became a real workhorse.