Yeah that's about right, maybe a bit harder than Divine Glory due to higher damage, but then Divine Glory isn't the hardest FG.
The thing is, no one wants to have another FG as hard as Rainbow, who most people wouldn't want to fight even if they could win. The chance is just too low to be worth the time, and you rarely win any card spins on him anyway. Vesta has a better chance of giving a card, but if the win rate is as low as Rainbow's, it may still be an auto-quit.