I'm looking to get 21's. Why not 21's? Is it the rougher or louder ride?
More wheel, less tire. Very short sidewall, that offers very limited damage defense due to no cushion compared to 19s and smaller. Potholes, and other road hazards can bend a rim in an instant. That is the penalty of ultra-low profile tire/wheel combinations.
I had the same experience as the OP with and Infiniti G35 Coupe 6MT. A pothole took out right front/rear rims and both tires. A pothole that a normal sized wheel/tire combination probably would have bounced out of undamaged. Rims $900 each, and tires $250 each. I'll never go larger than 19 again on any vehicle. I don't care what it looks like.