Unions represent their members' interests, that's the entire point of unions. It says more about the entire profession of policing (i.e. that there are so many terrible officers that the not-terrible ones can't do anything about them) than it does about unions, to be honest.
There are two unions representing metro train drivers in London, and I have little respect for RMT since they've gone on strike to defend a driver who was sacked for turning up drunk to work[1]. The other union (ASLEF) seems more reasonable.
When an issue is important to both unions (e.g. pay for nighttime services) they wield considerable power. If only RMT strike, there's significant disruption but trains still run.
[1] https://tfl.gov.uk/info-for/media/press-releases/2015/februa... / https://www.theguardian.com/uk-news/2015/mar/07/london-under...