Among the criticisms I've heard ...
Sometimes market forces can get you adequate conditions and pay/benefits without the need for collective bargaining. For instance, software engineers tend to be well paid even without needing to unionize. So if you don't feel that you need a union, but you're forced to be in a union (and pay dues) against your will -- as happens in many industries -- you may feel those dues are taking money out of your pocket unnecessarily. In fact, the union leadership might actually be making decisions that benefit them or the members collectively but that don't benefit you individually.