A union would only work if there is one Major Organization also. You cant have the best fighters spread out over Bellator, One FC ect and have a union. Because the only one that is being held to the Union standards would be the UFC because no other organization has the health plans, steroid testing policies, PPV revenues, marketing machine, fighter support services, extended training facilities were fighters can bring there camps to vegas during press tours and train at the UFC gym. The UFC info structure is so much more, the union would use these items as a basis of what an organization is required to due for each fighter (contract) along with pay increases and retirement plans ect.
So with no other organization having or even able to match the UFC offerings under a union, it is a sense would take revenues away from the UFC that it doesn't from the other organizations. This would not create a level playing field for the organizations across the board.
Unions work in the NBA, NFL, MLB because there is one major league.