The vast majority of mountain bikes sold today use hydraulic disc brakes and I am not reading a lot of articles about mountain bikers stranded out in the woods due to brake failure.
I've never had a problem with my brakes in thousands and thousands of miles they are pretty bullet-proof.
My SRAM brakes use DOT 5.1 fluid. My Shimano brakes use mineral oil. Looks like the HY/RD system uses mineral oil as well.