Racing? Of course it will be more difficult, I'm not sure how that's even debatable. I timed a team mechanic changing a rear wheel on a bike at the TDF, and it took him 12 seconds. That was the rear wheel. I challenge anyone to prove to me you could get a disc wheel swapped in a time that's even in that ballpark. And when you're talking about a rider who has to bust his rear to catch back up to the peleton I do not see how it doesn't become a major issue.