My ability to negotiate salaries with my boss is one of them.
The scope of my job (some unions get to decide what you are allowed to do).
Who should get fired (teachers unions make it impossible for bad teachers to be fired, so students are forced to put up with them)
If you should be allowed to work. Some unions forbid you to work during strikes. Often by force and by threat. Some unions manage to lobby for regulations that make it harder to get into a job. Then you need certifications and/or degrees to even consider a job.
Unfortunately many use the "democratic union" argument to control your ability to make personal decisions. Unions that get to channel power from the government (commonly through regulations) can easily abuse their power. When unions are not voluntary they easily become HOAs for jobs