I've never belonged to a union, so I can't really put forth any sort of educated opinion...but I will say this:
After being discharged from the army after a nine year stint, I found that I had no skills that were directly transferrable to a civilian occupation, so I ended up doing construction, specifically framing houses, which I did for about three years. I don't think that any of the bosses of these framing crews had any inkling what was written in the Manitboa Labour Code...during those three years, I worked on average 10 hours a day. Not once was I ever paid time and a half for my overtime...also, I had a problem with how I was paid, which was on the last day of every month, which again is not allowed under the labour code...also, he was a degenrate gambler, and there were a few times that I had to wait even longer for my paycheck, because he had tossed the payroll into a VLT...I brought up the labour code infractions to my boss, even provided documentation from the labour board, and he said that that's how things work in construction, and that if I wanted to keep my job, it would be best if I didn't push the issue...well I did push the issue, and got a raise out of the deal, but it still didn't sit well with me that he was playing fast and loose with laws that were enacted to protect me from crap like that...
So I quit, and got a job working for a company that did fire and flood restoration for insurance companies...I thought that things would be better there, but they were just as bad, and sometimes worse...because of the nature of the work I was doing, many times I was forced to work in an unsafe workplace, as we first had to make the site "safe" to work in...we never received any pay incentive for putting our lives on the line during these times...also, we never received overtime pay there either...one fo the boys went to the labour board with a complaint about this, and a few days later we had a staff meeting where our boss stated plainly that he could not afford to pay us extra for overtime, and that if there were any more complaints to the labour board, he would close his doors and put us all on the "pogey" line...he then made some hollow promises about profit sharing, and mandatory vacations (unpaid of course), none of which I saw come into effect after 6 months...so I facilitated my own lay off, and went to school so as to get out of the construction industry...
Having said all this, I think that if there were a union in place, I would have gladly paid my dues to be protected from some of the crap that I was forced to put up with, and then I might have still been swinging a hammer today...however, labourers (tradesmen with no papers) have no protection from being taken advantage of by unscrupulous employers...and I've not met any employers in the construction industry that have had any scruples...I guess having scruples would prevent them from vacationing three times a year...
Oh yeah...and the Manitoba Labour Board is a sick joke...