I put on a front disk and I thought no more worries until coming down from the mountains where I live, I discovered something that felt like brake fade. I touched the brake and it was really hot, enough to color the disk.
My reason for putting on the disk brake was that coming down the hills, I was afraid that the friction on the rim from my old front V-brake was enough to really heat up the rim.
Bicycle brakes are just that, designed to stop a normal bicycle. My engine and touring equipment adds another 50 pounds and I'm a big guy about 210 so I am hard on brakes. I also like to ride pretty fast down some pretty steep hills.
I have a drum brake in the rear that works pretty well, so I'm happy with that but I'm thinking in addition to the front disk, to put the V-brake back on in the front too. This way I always have a fresh brake in the front to use depending on conditions.