I think it’s fairly obvious that advisors are in a power to be abusive, and though many aren’t, a large number are. And oblivious advisors can be as hurtful as malicious advisors. If a student has undiagnosed depression, it may show in their recommendation letters in the form of descriptions of laziness or lack of enthusiasm. Advisors have huge career-altering power. A set of nasty letters from a former boss can sink your chances at good jobs, and there’s really no system to avoid this if you happen to accidentally piss someone off.
On the other hand, some professors are very keen on their students' happiness and treat them with respect. I've seen professors who go by their first name and invite their lab to gatherings at their house as well as professors who want formalities (including scheduling appointments with a secretary to even have a ten minute talk) and aren't interested in their students beyond a working capacity.Go over there and read -- it's really good.