I suppose you haven't really took time to actually understand where Rumble's difficulty comes from.
Of course he isn't on Azir-level of difficulty, but that's mostly because all stuff you have to learn about Rumble is subtle, and it's those subtle mechanics that differentiate experienced Rumble players from less experienced.
Proper heat management, positioning, avoiding enemy's melee range while keeping them in your Q range, lane management (last hitting, harassing the enemy without pushing), consistently pulling off ults that do more than damage one person for 2 seconds, the fact that he's a squishy melee/mid-range caster based purely on DoTs and with essentially no mobility spell.
Just because it's not shoved up people's faces doesn't mean it's not there.