No different than if a union doesn't kick out shitty union members no? The same still holds true. Anything a bad union does, a bad boss can do, at least with a union you have a vote.
Unions work much better in public sector where they can't go out of business. Police unions, prison guard unions, teacher unions etc. I wouldn't say service or accountability is that great in these systems.
When you're competing with a Mexican, a Bangladeshi or a slave in Thailand all working for a pittance it doesn't necessarily matter how hard you work. Unemployment or poverty wages are your two choices.
Police unions and prison worker unions are more effective because they tend to aid rather than threaten capital. Teachers unions by contrast are not, which is why most governors declare outright war on them.