"HR does not operate with your interests at heart, do not talk to them."
-- Anonymous
-- Anonymous
Frankly, if you are to the point where HR is even in the conversation, then you really need to have a large financial incentive to stay around. The reward better be worth the risk.
Ever had a complaint made against you? Possibly - maybe it was quietly filed away. Ever wonder if people are paying attention to when you come into the office? They are.
I went through an outsourcing at my last job and for nine+ months the only people that knew it was coming was executive leadership, the CIO, and his HR staff.
HR is risky because they have hidden information in spades. The interests you think align might not after all.
Scenario A: your job sucks, and you don't see a future there. Your boss constantly undermines you, doesn't give you credit, gives you a workload harder than any reasonable employee could work.
Scenario B: your company is doing something illegal, and you pointed that out to your manager. Your manager started undermining you, not giving you credit, uninviting you to meetings about your work. Now your job sucks and you don't see a future there.
In Scenario A, you should just leave. HR isn't going to fix the problem, they're just going to throw their hands up, tell you to work through it with your manager, and then quietly label you as a difficult employee. You're probably on a track to be laid off or PIP'd in the next cycle, so might as well either learn to shut up and take it, or get out.
Scenario B, you should absolutely tell HR. HR won't do anything, or they'll make your life worse. But that's exactly what you need to have happen to build a case. Depending on the severity of the situation (i.e., how pervasive and illegal the activities are), HR's inactions could be worth hundreds of thousands of dollars to you.