Based on what support told me...
GM Izko:
Hello there,
Unfortunately, we are unable to provide a exact percentage of the success rate of Enchant Scroll of Forbidden Fate but it guarantees a safe enchant until +8 but using it from +8 going to 9 & 10 it will be not safe and have chance to fail. I hope it clarifies your inquiry.
BTW if you use it on a +8 Talisman and it fails, it will not affect your current enchant and it wont brake.