The issue that worries everyone is that when an issue arises, HR is almost always on the side of company management, not the employees, even when the better solution to the company (and, for the trivial amount it's usually worth, the ethical solution) is to support the employee.

I've seen this happen directly to my wife, and heard many stories from other sources. She had a new boss parachuted in who was seriously borderline abusive (screaming and threatening behind closed doors, etc.). She was (mis-)advised to talk to HR in confidence to start a documentation trail... and within minutes after she left the office, HR was in the CFO's office, the CFO took the manager's side without bothering to hear hers, and she was fired for incompetence within a month... after a stellar performance review only 2 months before the new boss started. (They settled out of court afterwards - not a good solution for anyone.)

Read the new posting by the former Uber employee, where the same manager had multiple sexual harassment complaints, but all were treated as "oh, it's a first-time offense so we don't want to ruin his career." (https://www.susanjfowler.com/blog/2017/2/19/reflecting-on-one-very-strange-year-at-uber)

This is why HR is really feared - they are not on the side of the employee in any case where it matters.