From the article:
As Pahl walked about Eurobike and Interbike looking at disc brake-equipped road bikes, he winced whenever he saw bikes spec’d with 140- or 160-millimeter rotors. Rotor size and type are vital when considering heat dissipation.
And Pahl said he’s doubtful small rotors—currently fashionable on road bikes—can dump enough heat on long descents to keep braking systems from boiling...
Further, there are questions about the mounts:
The static disc brake torque test for road forks required by the EN 14781 standard is more rigorous than the EN 14766 standard for mountain bikes. Road bike forks must carry an 8 percent higher load than a mountain bike fork and a road fork’s braking cycle fatigue test is 66 percent longer than for a mountain bike fork.
“I don’t know why the road disc fork testing is to higher loads and cycles than a mountain bike fork, but it does reflect higher road speeds and less tire slippage of a road tire on pavement,” Baker said.
But the key thing seems to be concerns about heat dissipation:
While road standard tests are for higher peak loads and greater cycle frequency compared with mountain bike discs, the heat dissipation requirement is the same. Road and mountain brakes must absorb 75 watt-hours of energy over a 15-minute period without failure to be legal for sale in Europe. To put this in perspective, a 200-pound bike and rider traveling at 25 mph dissipates 3 watt-hours of braking energy to stop, and no matter how many times they stop they are unlikely to overheat brakes.
However, the same rider descending a mountain pass hitting 50 mph in stretches between switchbacks and who brakes to 15 mph for corners uses 11 watt-hours to slow down. If the descent averages a 7 percent grade, add 2 watt- hours to each brake application. After braking for seven or more corners within a 15-minute period, the rider’s front brake could have absorbed much more than 75-watt hours of energy required by the standard.