Brake rotors are prone to overheating especially descending for miles at high speeds. It's a much higher temperature accumulation on a road system as opposed to a mountain system. A warped and pulsating rotor eventually can result. I actually have a warped rotor on my mountain bike which is not a big deal on trails, I really don't feel it. When I have to go over asphalt the pulsations in the brake lever are really annoying. If that was on a road bike on a long decent or actually any braking application that isn’t going make for a happy rider. It happens in the auto industry all the time and it will in the cycling industry.
Disc brakes have a tendency to drag, if athletes even think it's going to affect power output they'll shy away. I know some riders that are conscious of wheel hubs that drag a little and complain about it.
Brake squeal - on the road oil and chemicals sit on the asphalt and when it rains oils will kick up and can contaminate the pads causing squeal in some cases. Again on a mountain bike not really a big deal, but on the road and you'll get a lot of upset riders.
With the Braking power centered at the front axle a lot more stress will be placed on the forks. Think of the fork as a lever or pry bar putting pressure on the head set and the rest of the fork. Hopefully fork flex will not become an issue. I know the forks are beefed up but these are lightweight, generally speaking racing bikes. Brake torque could also be an issue on spoke twist. I've heard of a few riders already thinking their headset was loose when it turned out to be flex in the fork during heavy braking. Again in Mountain bike terms the forks won't flex, they're massive structures designed to take a pounding. There's no comparing the two.
Some people think because they work so well on mountain bikes they'll be great on the road. There really is no comparing the two as the engineering obstacles are worlds apart.
I don't mean to demonize discs on road bikes but just be aware of some of these issues that will pop up, just how often no one really knows yet. I'm certain over time many issues will be solved but until then I'll hold off.
|Simple adjustment after 6000 miles|