They've done a very good job before, so good almost nobody heard about it. I would not bet on ISIS in Saudi Arabia.
If they're to succeed they'll have to get into the Saudi royal family because they're not going to supplant it. Bore from within. There have been several who have gone down that road never to return.
There was a 27 year old prince of the royal blood with ties to al-Quaeda who, shortly after 9/11, was found dead in his car in the middle of the desert. Cause of death: drowning. (Saudi Intelligence does things with such style!)
Based on experience, given you screen name?
SA has used the relief valve of allowing their ‘youths’ to flood the battlefields of Iraq and Syria. Gitmo is full of them, as was Abu Ghraib.
They don’t mind them going out and killing whitey or killing the Shia. But, they don’t want them back in their country.