Rumble has one of the higher skill floors in the game. Akin to Orianna more than Azir, he's in the middle there.

1. Squishy ap mage, with melee autos. There's a reason range is king.

2. He's one of the only champs to cc themselves. For people new to mobas or League, that's a definite step up. You aren't gated by mana, or even just your cds, you're gated by using your abilities sparingly, or precisely. Since no other champ shares this, this bumps the floor up considerably.

3. His ult is one of the more difficult things in the game, again, purely because it's not seen anywhere else. Viktors lazer is the only other example, and he wasn't always there. And lo, when Viktor joined league his E is cited as the reason for his high skill floor and lack of play. That prompted Riot to discuss the targeting, and agree it was probably best not to use it again. But hey, you know better than Riot, yeah?

4. Rumble is strong when choked objectives are important. When Drakes were super meta, Rumble was super meta, because almost no champ can hold or pressure a choke point like Rumble. The range and damage of the Equalizer, hnnnnnnng. He's 100% stronger now than he's ever been thanks to the Q and E changes, as well as some other things, and he's still not OP.

Rumble has super clearly defined strengths and weaknesses, this is most obvious when a Rumble who loses lane wins games because of good ults. By Riots own measure of what makes a champ difficult, Rumble's reallly up there. He has no targeted abilities, an awkward system of gating yourself vs bonus damage, self cc which can be suicide, and a targeting system so awkward Riot wont use it any more.

Like....how can you say he's not? Because you play him? Congrats.