I'm not exactly sure if /r/askhistorians is the perfect spot for this question, and it's actually already been well hit on by this thread in /r/asksocialscience. https://www.reddit.com/r/AskSocialScience/comments/1qy8p0/do_shia_muslims_face_discrimination_when/

To sum it up: they have faced some discrimination over their religious views, however, the Saudi economy benefits greatly from the Hajj, so denying it to such a large portion of the Muslim population would be contrary to 'good business'.

What's the atmosphere like at ground level? Do Shia and Sunni tolerate each other at the hajj? Do fights break out? Another random question: Do members of ISIS go? And if so what's the mood there given their extreme intolerance of other 'heretical' Muslims?