I am not a very strong believer in Ulfric fighting for Ulfric. Anyone could say that for man who is trying to overthrow and old dynasty, right or wrong. But his undisclosed history which is a spoiler and I do not tell here gives you some big doubts. I was a Stormcloak sine the Imptials tried to kill me. Ulfric's goals of freeing Skyrim from Imperial and Dominion yokes has already been achieved, Talos worship has been restored, and no one gains from a prolonged conflict. This may seem like a super nice (almost too nice) move by Ulfric… At the insistence of some of my generals and shield-friends, you are all invited to present yourselves before me in the Palace of Kings at Windhelm. The Imperials are trying to maintain peace and Ulfric is too dumb though.

You just made my first playthrough as a stormcloak supporter much easier. If Ulfric is not racist, someone else will be, and Skyrim will be closed off to …

Ulfric Stormcloak certainly cuts a dashing figure as a romanticized freedom fighter, but the means by which he came to power raises a few questions concerning his true character. I never once saw a reason to join the stormcloaks, because I always felt like it was so obvious that Ulfric was doing it purely to empower himself with all the motives to veil that, but it's obvious that there's more to him than meets the eye. Also the Thalmor are trying to destroy reality so the survival of the Empire is pretty important.

The Imperials are trying to maintain peace and Ulfric is too dumb though. Some of Ulfric's military commanders and Jarls will undoubtedly feel betrayed, but the devil of the rhetoric, propaganda and policy is in the details. On the Stormcloak side, you get to take Solitude, where Ulfric gives a speech about how he is still going to let the Moot decide on who will be High-King/Queen and he's even letting Elisif stay on as Jarl of Haafingar.
This is similar to another question I answered: Did you choose Imperial or Stormcloak when playing Skyrim? The Thalmor were relying on Ulfric Stormcloak to start the civil war, causing great losses throughout Skyrim and further fracturing the Empire, who is the biggest threat to the Thalmor. I never knew all those things about him despite playing the game so much. In that answer I heavily supported the Imperials.

My name is Ulfric Hoagsson, of the clan Stormcloak. My father was Hoag, the Bear of Windhelm named for the old High King Hoag, and was a great jarl of Windhelm worthy of his name.