In general, it's a tired decades-old anti-labor talking point.

However there are many cases where it's hard to fire underperfoming unionized employees. And there's always the possibility of the union playing a kind of long game with management where they are willing to trade away the possibility of rewarding standout individuals in order to preserve the baseline.

I've never seen a union contract that didn't have a clear path to firing someone. I've worked in a union and in HR labor relations. My experience is that 'hard to fire' stories usually boil down managers not understanding the union contract, not enforcing the union contract until its too late, or wanting to enforce the union contract differently based on the employee.

Ditto. 100% of the times I’ve seen that happen, the problem was created by bad management and often perpetuated because then they don’t want to have to explain why it wasn’t dealt with earlier.