I haven't been on a bus in the mountains of Peru but I have been on a couple Ecuador. That was one of the most terrifying experiences of my life. Going both up and down the mountain we were flying along roads that were barely wide enough for two vehicles, only a few feet from sheer drops of hundreds of feet. Often we would come up to blind corners where the bus would slam on its brakes before doing a U-turn while honking its horn, with the tires going so close to the edge that I couldn't actually see the ground even when pressing my face against the window. A few times we did that and another car was also approaching and had to slam on their brakes to avoid slamming into the side of the bus. Being a bus it would have obviously destroyed the car without doing too much damage to the bus, but I imagine it could have also pushed us just enough to tip us over...

That trip was incredible, but if I go again I will find another way up the mountains. There was the option of a multi-day hike. In hindsight that was probably the safer choice.