Yes, I had a game where I farmed almost 200 minions too, however this farming was done latemid-late game because your flamethrower does not get a lot of minion kills early game. And as I said it pushes so much early game what will open you up for ganks.

As for his late game, he is very tanky, however in mid game he excels much more than in late game.

I have seen many Rumbles in my past games, all who had like 20-30 minion kills max after early game.

Dont agree with your "cons", i´ve just played rumble...he owns in late game, he´s a very tanky champ, and he isnt a terrible farmer, considering your Q and E skills, and using well your passive makes him a very good farmer, i killed more than 200 minions.