Concentrated power in the hands of capital has had utterly devastating effects on millions if not billions of people. It has easily cost 7-digit numbers of lives. Yet in general, capitalism is taken for granted as a good, discussions of problems always get tons and tons of "yeah, but even if it did that bad thing here's a bunch of good that outweighs it", and at best people suggest only very moderate restraints as a remedy.
Putting even small amounts of power in the hands of labor... does not get that kind of pass. Instead, the focus is almost exclusively on harms, flipping the story to "I guess they did some good, but..." and then bringing up criticism after criticism, either implying or outright saying that unless labor unions manage to somehow be perfect angels at all times it won't be worth allowing them to exist.
What's even more ironic is that a labor union is just a legal form that pools a resource, for the benefit of those who provide that resource, in ways that wield more power than any of them could individually. The only difference between a labor union and a for-profit corporation is the resource being pooled.