You can see it today with police unions being the biggest hawks blindly defending any police abuse and protecting them for ever being fired. This elephant in the room often gets ignored in public discourse. And the police aren't the only ones.
I grew up in a small town where unions were used like mini fiefdoms riddled with nepotism where you had to know someone to get a job. Theres also a long history of racism in unions, the original forming of them in the US were to exclude low wage black workers from 'stealing' white jobs.
It seems to work well at first before slowly morphing into a rigid protection racket for current employees, locking a company down to its current structure (which only makes sense for a few slow moving homogeneous industries like factories - until of course they simply automate or move to Asia or shut down).
It's also notable people always use historical nostalgic examples to hype up some golden age of unions for jobs that no longer exist for some reason...
They've also long been covers for corruption and back room political influence.
I personally believe unions are long overdue for reform and modernization. I'd maybe support something like unions-light, but today they always come with tons of other baggage like preventing bad workers from being fired, adding tons of bureaucracy and hoop jumping, crippling companies ability to adapt to market changes, forcing people like comedians and small indie bands to hire 30 different people for individual jobs (like paying a guy $40hr to pull a cord to open the curtains and nothing else which happens in NY clubs) creating inefficiencies which hurt company growth and customers and therefore employees/economies/taxes, completely shutting down small businesses who simply can't afford to support unions, not applying well to companies with a variety of different job roles, etc.
It's like a sledgehammer when most people just want to make more $$.