Well, HR staff are people like any other department, so as with all people there's always potential for asshole behaviour even in a company that generally encourages HR to be employee-friendly, and potential for someone to do a nice thing that's technically against HR policy in a shitty company. Not to mention that the abstracted layer above - whether the company has employee-friendly or employee-hostile policies when it comes to HR team - is also ultimately down to people with the same potential for good and bad.
"HR work for the company not for you, don't trust them" is a reasonable general rule considering the average HR department especially in large companies, but it's not a guarantee that all HR people, or even all HR teams' policies will be evil.
Agreed; I believe the tipping point is where the company becomes so large that you no longer know, or at least are familiar with the people you're working with.
I expect you are much more inclined to try to find a workable solution if the case at hand is Dave in accounting whose kid attends the same soccer practice your kid does than if he's just employee #628481.
"HR work for the company not for you, don't trust them" is a reasonable general rule considering the average HR department especially in large companies, but it's not a guarantee that all HR people, or even all HR teams' policies will be evil.