Without wanting to be pedantic, what are they?
I am/was in the same boat as you. Underwhelmed by his signing, but glad he was so instrumental for a while. But I never saw him as more than that Bearded Genius Valencia conundrum pic ("You know what he's gonna do, but you can't stop him")
He was quite a good crosser, but it was mainly dinks to the far post or smacks across goal. There was never any Beckham-esque indication he knew exactly where he was putting it. He didn't take free kicks. He wasn't a brilliant finisher in terms of placing a shot, he'd just whack it. He wasn't skilful or a masterful passer. He is and has always, always been just this very fast, very strong, very right footed player who was playing extremely well. To me at least.
It may not be a case of people "sussing" him out, it may be fitness, or confidence, but the case still seems to be that he can't do that one thing anymore. And as such, is utterly useless.