I am not a sun god main (I merely used him as a secondary for a while), so it's possible I've missed something here but, to me, ex command grab is one of the best (maybe the absolute best honestly) armor moves to option select in the entire game. When I play sun god, I basically mash this during everyone's pressure. As with all OS armor, it doesn't come out of there is no gap, but it does come out of there is a gap. However, unlike most other moves, it's unblockable, so people can't bait it out by stopping their pressure, blocking, and punishing. It's also fairly hard to armor break. So for a lot of characters the only options they have are gapless pressure and randomly jumping during their own offense, which opens them up for big combos at times when they should be at an advantage.
So to respond to the question of when you would use sun god - I would say vs characters with strong pressure that have gaps.