Former TL here.
My father was with the CWA union until the day he died. Toward the end of his life, his job was eliminated and he was forced to apply for a new job in the company against all of his other coworkers. He wound up getting a position, but at a $5 an hour pay cut.
I worked as a cashier at a union grocery store about ten years ago. Union dues were twice your hourly wage once a month (so if you make $10 an hour, you'd pay the union $20 once a month). We were ineligible for a pay raise until we had reached a certain number of service hours based off of your current rate of pay. Sundays didn't count towards service hours, so you'd make an extra $1 an hour (depending on your location, I guess that varies anywhere to an extra 25 cents to $1). We had to punch out for our 12 minute breaks. In order to avoid giving us unpaid lunch breaks, we'd be scheduled to work for 7 hours and 45 minutes. Any time I hired a new TM from a union grocery store, I'd ask them about their experiences. Doesn't sound like much has changed in those ten years: the staff was still underpaid and not compensated when they learned a new skill or took on extra responsibilities because they weren't at that magical number of service hours to warrant a pay increase and Sundays still don't count towards service hours. We weren't actually given insurance cards; if you went to the doctor you had to bring in the bill and fill out some paperwork and send it in to someplace and they'd take care of it from there - very complicated and a lot of work just to pay for a routine check-up and I have no idea about any kind of emergency situations. We also didn't get holiday pay for working on holidays. Instead we had to be there for our scheduled shift before the holiday and our scheduled shift after the holiday and we'd get 4 hours at time and a half pay. So if you worked your shift on Easter vs someone who didn't have to work on Easter and you both worked your scheduled shifts correctly, you'd both only get 4 hours of time and a half pay.
If anything, I could see a union come in and eliminate more positions rather than protecting the people who are there. There's no reason for there to be any more titles on a team than just a TL and an ATL, so all buyer, supervisor, team trainer, etc. positions would be cut (with a pay cut to boot) and the responsibilities would just fall back on the TL and ATL (which would mean even less time on the sales floor for them).