Hydraulic brakes can get so hot that the fluid will boil, and that is not good. While air brakes do not have that problem. Nothing to boil out, just air.
I would take air over hydraulic anytime. Since airplanes were brought into the discussion as to using hydraulic over air brakes. On airplanes brakes are applied just for a few seconds, vs a train going down a mountain which could take a long time. So then why do trains which are a lot heavier use air brakes. I worked with them for 42 years and can tell you all about air brakes. Because they are better.