Here's my perspective after towing a 10,000 lb fiver with a 2006 Chevy 2500 HD for several years, including one trip from Indiana to the Grand Tetons, Yellowstone, and back home via the Black Hills. Could I do it? Yes. Was I crazy? No. Did I have any problems? No. But, I will tell you that it was a lot of work, and especially when driving any significant grades at altitude. It was what we had at the time, and I made it work. But since then, we've upgraded to our current 3500 D/A and the difference is night and day, even with a heavier trailer (13,000 lbs loaded). Can now use cruise control, and don't have to creep over the passes with my foot buried in the floorboard to just maintain 20 MPH. Oh... and did I tell you my MPG increased by some 40%?
So would I do it again? Yes, if that's the setup I have and am not in a position to upgrade. I didn't feel unsafe, just under equipped.
My experience. Others may have a different one.