Designed to have same stopping power in dry as a well set-up caliper brake (ie, limited by tyre grip) but they have even better modulation and work as well in the rain or dirty conditions. Why wouldnt you want that?
because they are horrifically expensive, are made by Avid which ime means unreliable crap, for the purposes i use a road bike my brakes are good enough, i have no interest in buying a new bike, crashing is not an option!
and if you notice Sram are hedging their bets, they have released a hydro caliper so they aren’t sure which way the market is going.
Sram electronic is on the way so whatever you buy will potentially be out of date very quickly, which means spares may be an issue.
Finally, have i ever been riding and think, you know what i could do with more braking on the road bike? no. MTB is very different, did i ever think more brakes, no, but did i think increased mud clearance, yes!!!