I was stationed at the US Embassy in Jeddah, Saudi Arabia 30+ years ago. I had a good friend who was a CIA paramilitary officer. His job was training the Saudis on the arms that we had procured for them. He also taught them how to jump out of RSAF C-130s. I hope that they have improved over the last few decades to where they can successfully engage ISIS and AQAP.
I think they’re a little bit better and my guess is that the typical (Saudi, Egyptian, Jordanian) Sunni animus for all things Shiite will give some impetus to their military efforts. My personal feeling is that if they’re fighting each other and we don’t have to do it, all the better. As long as the KSA is not destabilized because IMO anything that replaces it will be worse.