I have to say, I felt like a real hypocrite joining the Legion after finishing the Dark Brotherhood questline but I felt like it was something I had to do to make the game feel complete and I just couldn't force myself to join the Stormcloaks.
Ulfric is a power-hungry and savvy man who is manipulating the emotions of an outraged population and turning their attention away from the real enemy, the Thalmor, and towards the only thing that can save them from it, the Empire. A free Skyrim is just another part of the Aldmeri Dominion, in my mind.
Ulfric is using the anger at the White-Gold Concordat as a means to his coronation, whether or not he truly believes freedom is what's best for Skyrim. He's playing politics.
I can't join a man who quite unabashedly pushes his own cause to no end, even in peace talks, so he can gain more power especially when the results of his victory wouldn't be good for most people who aren't Nords. Not to mention my earlier point, the Aldmeri Dominion would smother Skyrim and use the land as another front against the Empire for when the Great War inevitably restarts.