From a balancing standpoint, this would be completely ridiculous. Rillaboom gets Grassy Glide and an Ability that gives said move automatic priority. Give it a Life Orb or Sitrus Berry or the such, and it can and will go on a rampage.
+6 252+ Atk Life Orb Rillaboom Grassy Glide vs. 252 HP / 252+ Def Toxapex in Grassy Terrain: 441-519 (145 - 170.7%) -- guaranteed OHKO
That's a an OHKO against one of the tankiest Pokemon in the game.
It also has the chance to 2HKO Heatran and 3HKO Corviknight/Ferrothorn (with Grassy Glide! Imagine Drain Punch!). Isn't that ridiculous? How does this make any sense, balance-wise? It doesn't. Game Freak doesn't care much about balance, but it does care a bit.
It also doesn't really have much of a belly. While it has a literal drum, and its entire thing is drumming, look at that chest. Kind of small, no?
Hope I helped!