The most common gripe I hear is that because not all labor is similar[EG: Healthcare vs Mining] there is no easy way to make sure that people are putting in the same value into the system. For people that already mistakenly believe there is a large group of "lazy immigrants" draining resources in the current system, this is a deal breaker. The internal logic isn't too much to swallow its just a hurdle to get people to sit long enough to hear the answer. They would rather fight over the crumbs than share the whole because somehow thats more "fair".