Skarner has the same problems he always has had since his s2 days of dominance, only further exasperated by meta changes and new champion kits. He still has a feast or famine issues, if he doesn't get ahead then he falls extremely far behind in usefulness, and struggles to get back in.
Furthermore, his ganking is very one-dimensional, and the mobility meta in both laning and jungling means he has an even harder time ganking, even with his slow. He has worse early clearing because of a complete lack of sustain, has average clear speed and average dueling, and falls off extremely hard as the game progresses. Even with his ult and passive stun, it requires him running straight at somebody or sticking on them for about 3 or 4 seconds, respectively, to even use them, all of which are hard to do against high mobility or high disengage.
It's not that he has nothing going for him, but that he doesn't have enough going for him to make him worthwhile for any specific thing. Jack of no trades, let alone a master at any of them.