I view unions as something that's only essential in the worst case scenario. If you truly need one then I fully support that, but they come with a huge cost to innovation. It's important to not forget the trade off.
Hate to break it to you, but "unions are ok but only under specific circumstances I describe" is anti-union.
They're always 'OK' (I'm not saying they're immoral) but they do come with a cost (to innovation, to efficiency) that should not be ignored.