McDonald's Mr. Floersch the managerial turnover was at 20% globally while that of the crew members averaged between 80 percent and 90 percent. [0]
That high turnover means that low paid employees are much faster to leave. Some don't even show up to their first day of work. It's also selection bias sure. But if you talk to anyone who manages low paid employees, the primary skill set they have is interpersonal relationships with their staff, or at least the good ones. The fixed costs are very high to hire someone, regardless of salary. You have to pay a recruiter or even spend time looking through resumes, running expensive background checks, interviewing etc.
Now consider the Goldman Sachs investment banking analyst. They get paid extremely well and get treated like crap. Do they have a "direct relationship with their employer"? Hardly.
If you're making $10 an hour its a lot easier to find another employer paying you a similar amount than it is if you're making $200k a year.
[0] https://ukdiss.com/examples/high-employee-turnover-at-mcdona...
Hell, I used to be a dishwasher then line cook at a Friendly's (east coast family restaurant, primary PA/NJ/NY/CT/MA) for two years and by the time I left for college I was the most senior person there who didn't have a key to open the building. And two of the three cooks who had keys were still in school, one HS and one college.
Managing a restaurant is super demanding work but it's not that far off from median salary. It's not as much of a dead end as people think it is, especially if you start at the bottom and work up.
I don't think that's necessarily a characteristic of a union shop and if it is, those are probably minimums.
For example, professional baseball players and are unionized yet individuals can still negotiate salary.
It would more likely be something akin to a teacher union, with teaching being in between performance athletes and uniform workers. And even there I don't think individual teachers have much power to negotiate salary, but I could be wrong.
Unionizing can make sense for other types of jobs as well. The person I responded to mentioned their preference for having a direct relationship with an employer. To me, that means other types of places than warehouses because I doubt very much that many warehouse workers have a relationship with anybody above their direct supervisor.
A warehouse employee is not likely to have any kind of relationship with anybody above their supervisor. They can only bargain collectively.
I don't think its as clear cut as "in your interest". I certainly wouldn't vote to unionize my profession and I would even go as far as avoiding unionized fields. I prefer to have a direct relationship with my employer. I stay away from organizations that have a table that will tell you how much you'll make after N years. It's just not a dynamic place to work. Or at least that's my impression when dealing with unionized groups in large banks.