From what I can tell, a "bad relationship" with employer particularly in competitive tech fields is a direct result of poor performance by the employee, which actually seems reasonable. I don't think it makes sense to introduce bureaucracy with the explicit purpose of covering for poor performance, as that pummels incentives to actually perform well at work. It seems actively unfair to people that are doing their job well if a union is covering for people that aren't.

Of course, a union can offer other benefits, but the downside of devaluing good performance seems strikingly not worthwhile.