I don't see how the union was doing any worthwhile work for me back when I was making minimum wage at a grocery store. All I saw was that they wanted to skim hundreds of dollars off my meager paychecks every year.
You didn't understand what the benefits of collective action against a hostile employer are. That's the fault of the union for failing to explain what they do for members (unions are about much more than just wages). That doesn't mean there's no benefit though.
a union is not an unalloyed good for the employees. I also worked at a unionized grocery for about 3 months. At our shop, it was clear that the union was set up to protect the senior employees at the expense of the junior employees. There were about 10-ish people on the checkout stands during my shifts, 4 of them were employees who had been working there for 10+ years, and 6 of them were people who had been working there for less than a year. The senior employees worked maybe 15% of their shift when it was particularly busy, otherwise they'd have "meetings" in the break room or hang outside smoking. The junior employees were worked hard and burned out and left.
That has nothing to do with unions though, obviously. If you look at any organisation you tend to see people who have been there a long time without moving upwards or onwards are pretty lazy people who know how to play the system. They don't need a union for that.
A lot of people who are anti-unions appear to think the union is the cause of everything bad without noticing the same things happen everywhere else as well. Keeping shitty staff around is much more a function of employers who don't track how good their staff are than anything to do with unionization.
Also, forming an opinion about unions based on one job that lasted 3 months with 10 co-workers, the majority of who were hard-workers, is appalling. That isn't anywhere near enough information. You really need to try harder.