Hal, in my decades of work the only times I've ever been screwed over
was by the union in a closed shop. The AWU wanted me sacked in one job because I worked more efficiently than their union rep, and later in another industry where the union decided who got hired or not the pay rates were so bad I left a job I needed a special certificate for to work as a machinist in a factory for 25% more money. The union fees were compulsory and high, and what you get after that wasn't much at all, but they were virtually the management of the business and the union leadership got a cut if the profits as the union had shares in the business.
Once I went to the union while a member in the APS and got told to get stuffed as I wasn't also a member of the ALP, so I quit the union. Either they support members or they don't, if they don't then I don't support them.
I've had the bad luck to be in industries where the unions were fairly militant and so left wing they made Karl Marx seem conservative. The union management were more interested in pushing their communist / socialist agenda than what the members wanted or was good for them.
The professional associations I've belonged to at time have been useful and good, and the one very small union I was in supported the members, but none of the big ones every did.
And I'll leave all the crap the compulsory student unions did for another time. They were NOTHING like the voluntary ones that used to be around.