I'd be surprised if that was true. Unions are good when there is an "we are all in it together", but with tipping a great server can make a lot more than a bad one, so all the great servers will defect from the union for more money. The bad ones want a union, but they are also the ones management should get rid of.

There are ways for unions to work around this, but the fact that some members see themselves as better than others in ways that are measurable make such unions weaker.