It's important to remember that - unless you work directly for the owner or an executive appointed by the board - they're not your boss' advocate either.
If the company is worth a shit, they don't want bosses that abuse their power or make their subordinates miserable. Happy employees are productive employees.
We've rid ourselves of a few problem bosses that way. Of course, this only applies to legitimate issues. If a boss is causing people to quit, you've got a good case.
This is the part everyone misses. I worked in HR for a number of years and 90% of my job was telling low/middle level managers "you can't do that to your employee." (I wasnt high up enough to be dealing with c-suite level complaintants), 9% was recruiting and paperwork, and 1% was telling an employee "You did something potentially terminable."
Most people only seem to recall that 1% and then keep talking about how "HR isn't your friend/on your side theyre on the company's side." Which is true! But they also didn't see the 1000 times I slapped their managers hand because I was on the companies side not the managers. Unless your really high up your manager is someone's employee too. HR isn't siding with you manager for shits and giggles, there is a reason management won a complaint against you and it isn't "HR likes management better." It's that they framed your problematic behavior better than you framed theirs. Frame everything you report to HR as "this is why it's a liability for the company" not "I don't like x,y,z. So-and-so is mean."
Also remeber just being a bad manager (not doing something immediately terminable) isn't a firable offense. Yelling/being a low level dick for example may not be something deemed firable. One complaint isn't gonna e enough and ideally multiple people will complain as well.
... which makes sense, because the reason some actions have legal repercussions is that people have passed laws for the purpose of discouraging them!
We have sexual harassment liability laws because we expect that if we make companies have HR departments that tell managers to not sexually harass their employees, then somewhat less sexual harassment will happen than without those laws.
The law isn't just there to compensate victims, but to align the company's incentive ("we don't want to pay out a bunch of money") with the worker's incentive ("I don't want to be sexually harassed"). The company can avoid paying out a lot of money by not tolerating sexual harassment in the workplace.
It doesn't always work out that way, because corruption springs eternal; but I expect more nonconsenting asses would be grabbed if it weren't someone's job to say "don't grab asses in the workplace".
The trick is knowing how to phrase it so it's clear it's a problem for the company. They usually love SBIN (situation behavior impact next steps) so it's good format to use:
Dear HR,
On the meeting XYZ
My boss Bully McIdiot was screaming like a toddler at everyone that disagreed with him
This is preventing the free flow of ideas and Innovation and creating an »»hostile work environment««
However, the two things aren't mutally exclusive. Bad behaviour that risks reputational or legal damage to the company will make HR cross. Think about how you frame things when talking to HR
Unions are all workers friend, but they are not your advocate. If your salary is up to the agreed national contract and there is little they can do.
it depends on the country, and where exactly you work, but in many countries (ehem Italia) they are somewhat too comfortable with the company management to be effective at their job.
It is still true, at least in Europe. I mean, they're not actually trying to destroy your life, you know, but they're after the company's best interests. They might help you, and might make things not the worst they possibly can, because that'll give a bad rep, but they're not your friend.