Things unions often do, historically:
• Require employees to strike even if they don't want to / can't afford to.
• Prevent employees being fired, for any reason or under any circumstances.
• Harass or even force non-members to join.
• Get involved with extreme-left politics and use union fees to fund the union leader's own political views / ambitions, regardless of the views of the workers themselves.
• Segregate workers into types, and attack the workers themselves if they do the "wrong kind" of work. The attendant removal of any flexibility cripples working speed and causes great frustration. For instance imagine being ticked off or even fined because you wrote a Docker config or created a monitoring dashboard, and that's the job of devops only, not developers.
• Block any kind of labour saving automation. Imagine being told you weren't allowed to deploy a script that checked your dependencies for CVEs because that was Joe's job and he's been doing it by hand for the last six months, so he's going to keep doing it by hand for the next 10 years. Now imagine that any task that anyone ever mentions is immediately given to a human to do instead of waiting for it to be scripted, so effectively nothing can ever be automated.
• Losing interest in advocating for workers rights.
These examples may sound absurd but unions of all types across all industries have engaged in this kind of behaviour in the past. It's a power thing. Because unions use their power to fight the government they can often get laws passed in their favour, granting them even more power, giving them even less incentive to work for the workers instead of themselves, etc.