As an American, I think it is just decades of seeing the problems caused by unions. In the worst case look at our police unions, it is now pretty well accepted

As an American, I think it is just decades of seeing the problems caused by unions. In the worst case look at our police unions, it is now pretty well accepted that they are a large part of the problem with police brutality and killings here. It's now a growing mainstream belief that they should be disbanded, and after seeing time and time again how they protect murderers and abusers it's hard not to agree. In other industries the stakes are not life and death in the same way, but it seems like the same principles hold. If a small percentage of employees in an industry are corrupt or negligent, and the unions defend them and prevent them from ever being fired or facing any negative consequences, they can do a lot of damage. Also, in the US a lot of benefits that unions provide for their members go against the interests of broader society. Unions are some of the biggest opponents of nationalized healthcare, for example. As far as the upsides, I think unions were more useful when governments were much smaller and weaker than they are today. In that world, the upsides outweighed the downsides because people had no other protections. With how incredibly wealthy the US is now, a functioning government should be able to provide the benefits for every citizen that unions provide for their members, while avoiding the downsides of the fierce "us vs them" dynamic that unions can create which has ended up dividing society in many instances. (Obviously there's a big asterisk there on "functioning" government, but many unions don't seem to be functioning very well either as I mentioned earlier, so it's a tie on that front. If we can't get our large bodies to function better than they are these days, we're screwed no matter what).