it's likely not the case here but more powerful unions can block non-union scabs from taking the jobs of striking workers (in that this is usually part of the contract the union has with the employer). at scale, unions have a lot more power to affect things
but the point is it's not about worker skill when unions are at sufficient power levels
When unions get to be that size is when they get captured by organized crime and other interests, because then they're acting as a de facto government and susceptible to the same corrupting influences but without the (by no means perfect but far better than nothing) safeguards we put on governments in modern democracies.
This is also why corporate monopolies are a fiasco and need to be prevented as well.
The general rule is "prevent any one group from consolidating too much power". If someone's solution is "let our group consolidate a huge amount of power" they're admitting they're the villains.
but the point is it's not about worker skill when unions are at sufficient power levels