First off A UFC Fighter's Union is a no brainer. A union's primary responsibility is to the safety of its members. In a sport or profession that is inherently dangerous or injury prone a Union is a no brainer. WIth that being said I really don't think the upper elite athletes would make this happen. In MLB, NHL, NFL, NBA, the top athletes took a substantial initial cut in salary which I don't see happening in today's age. It is a wise move that is best for everyone in the end.
The main reason why a Union in the UFC is that Revenue Percentages will be made public. In most pro sports the players get around 50% of revenue, but in the UFC many are estimating that it may be as low as 5% or even alot less. We really would only know this if the numbers were forced to be known as part of bargaining with a Union. If the fighters were smart they would start a union, but the UFC and Dana White will fight it tooth and nail, and a lot of the initiators would be punished severely, financially. And is anyone really willing to put their necks on the line?