It's easy to repeat the old line of "unions protect bad workers," since, yeah, sometimes it's true. But it's still not quite as black and white as that.
I previously worked for a union shop along with this one guy that was the kind of person you think of as the stereotypical bad worker that unions unequivocally protect. My friends and I always made fun of how shitty this guy was as a worker and how the union would protect him if it happened. Time eventually came when he got caught being shitty and was fired. Sure enough, the union fought for him to get another chance and he was back at work a month later. We laughed at how we were right, and then we laughed again when he got fired again and the union again fought for him to come back on. We all kept repeating the same old tired line about how the union was going to keep protecting this shitty worker. Well, it eventually came that he got fired, but never came back. It never dawned on us at the time, but as much as we kept bitching about him being protected, the union eventually did hit a breaking point and eventually washed their hands of him. Could be easy to just handwave this away as anecdotal evidence and it certainly not being the norm, but I've since seen plenty of documentation across multiple unions suggesting that, yes, eventually some unions will stop protecting shit workers.
On a related note, one of my best friends who worked at that same shop eventually ended up working as a technician for a non-union company. While he and his direct coworkers were not union, they often got paired up with union workers when they were contracted out by their company to assist at union shops.
He'd call the union workers lazy because they wouldn't start work until 9, made use of their lunch breaks, and then always clocked out at the end of their shift, while he always had to arrive early and couldn't leave until his task was done. While the extra money he pulled from those longer shifts was nice, we eventually started hearing about how he was having less and less time to spend with his children, and less time getting to socialize with people.
He'd comment on them being pampered by the union with all the protections their contracts offered, but then we started hearing about how his company would do things like suddenly change the travel policy (as his job relied heavily on travel), cutting into his earnings and also forcing him to spend his own money on gas for traveling back and forth between jobs.
And the biggest kicker was that, he'd also complain about unions protecting bad workers (which again, we both experienced with the one job we both had together). Then one day came when he was made to work a 16+ hour work day, followed by another extended work day that was timed to basically only give him a few hours to sleep, and only then if he just slept in his work van. When he started his shift the next day, he got sloppy with a task due to his exhaustion. He got found out, and the company called him out on it. Despite being a good worker otherwise, he got fired without any second chances. No union to help him.
In that moment, yeah, he was a bad worker. No denial about that. Do bad work, bad things happen. But given the circumstances, it's easy to see how he could have avoided it in the future. Had he had union representation, they could have appealed to his record as a good worker and argued for him to get a second chance on the stipulation that he didn't do it again. Hell, had he had union representation, he probably could have avoided things entirely because he wouldn't have had to pull two ridiculous back to back 16 hour shifts with little break between them.
Sure, sometimes unions protect people that don't deserve it. It sucks, because, yeah, it gives an extremely bad name to something that otherwise stands for a lot of good in this world. That said, knowing just how dramatically downhill my friend's life went after he got fired, I'll happily take a system where sometimes bad people get repeated chances versus a system where a good worker gets none.
Can you join Unions from different industry than your own?
It's situational, but possible. I've worked with records for a textile company where the workers spent time organized under the United Food and Commercial Workers instead of the Textile Workers Union of America because there was already an established UFCW local in that town that was willing to represent them, which expedited the process that they'd have had to go through in order to bring the TWUA in and start setting things up from there.