The following is my personal experience being part of a collective bargaining union (OPSEU local 598), which encompassed a few hundred workers for Ontario Teachers Pension Plan, in Toronto Ontario, from 2007-2017. I worked in IT for the duration of my employment (although not all union members were IT - but a lot were.)

The good:

- An elected collective bargaining team negotiated for us every ~5 years, and came up with a collective bargaining agreement (CBA). This allowed the members to express their desires for what they wanted (although not all requests were brought up in bargaining, and had to be agreed to to be ratified), which were generally listened to

- On-call compensation was set out as part of that agreement, and was the most generous I've experienced in my 25+ year career.

- Members could file grievances with the union regarding work conditions, or unfair treatment of the workers (I don't recall hearing of this ever happening, but there were processes in place for it)

- Health benefits were good, if not the best I've seen

- You could get two pay bumps per year, one that all union members got that was set aside in the CBA, and another moving you up a spot in your pay band (but only if you were not at the top of your salary band)

The bad:

- Union dues, while not huge, were yet another noticeable deduction from each pay

- When at the top of your salary band, you only got the one cost of living adjustment per year. There was no automatic way of moving to the next salary band

- Getting promoted means applying for internally posted positions (which all employees can apply for), and successfully being hired in to that position. This is the only way to move up salary bands, and you could only move up one pay slot in the new band (as they overlapped between bands). This really limited upwards career growth, and meant that leaving the company was the only way to get double-digit pay increases (or move in to management, which was outside the union)

- Our CBA strangely didn't cover / prevent layoffs of staff (although other union CBAs certainly do - so this is just my own experience), so I was one of the 100+ members that were laid off when a new VP decided to outsource a bunch of our roles to Tata Consultancy Services in India. There were provisions in place given my seniority that would have made a more junior union members have to be laid off in place of me (so I could take over their role instead), however I opted to take my severance package as I was ready to move on.

So to summarize - unions are definitely a mixed bag in my experience. I can appreciate the good they can do (and different CBAs will result in wildly different experiences), but from what I've personally seen, they generally function to treat all workers in a similar way: not rewarding the best, and not really punishing the worst.