Without suggesting that there were any related safety issues, during the early Scenic operations of the Cumbres and Toltec, Bob Keller & Co. used a couple of IH School buses, for the Chama/Antonito transit.
Made for an interesting trip both up and
DOWN the grades.
Unlike the modern Mountain View Van Hools that can get almost all braking effort fronm the engine retarder, I'd wager that the Scenic buses devoured a lot of brake shoes.
I imagine that some of the pre-WWII journeys on the Rio Grande Trailways Brill buses must have been quite adventurous considering the conditions of the roads and the design of the coaches.