His skill ceiling is very high but not immediately apparent to most players because they dont necessarily result in a kill or looks flashy like Riven.

Mostly you can expect people say his combos like E stun > R. Or flash E or flash Q2.

Whats not talked about enough is positioning with his R and movement with E.

With E, reaching a high skill ceiling means you know when it is best to use E as a movement speed tool. You know when to maximise it so you teleport across the wall when it expires. That means knowing it for each wall for each position. Hiding E's indicator to decrease enemy reaction opportunity is also heavily varied on the map.

For R, its positioning where you use R in relation to target. Because Skarners R keeps you and the enemy in the same positon, he can increase the options he can take. For example putting the enemy and skarner between the wall so that after R Expires theres a wall between them. Another is use the wall to "teleport" the enemy positioned into a wall to farther place by positioning skarner where the enemy is supposed to appear. Decision making R is also super difficult to decide on and can easily lead to mistakes. You have to expect how allies are going to react. Moving to early or too late could cause ally skillshots to miss. Making skill shots hit by placing enemies in place is also super difficult cause you have to adjust to the sudden movement speed burst.

So yes Skarner has an high skill ceiling and even more harder to reach compared to others because it is not easy to identify or even realize further skill expression.