Care to go into more detail? I know damn well Iow level illusion wouldn't work on a dragon. I hope at least. It's practically impossible to sneak up on dragons also from my experience with my stealth archer that I retired from at level 26. Might need to pick it back up if it ever stops feeling so boring.
As far as I can tell, Illusion spells don't work on Dragons at all, no matter how high the skill is or the perks you take.
You CAN sneak up on a few dragons, but only those that are pre-spawned in places (such as dragon perches). However, even with a 30x multiplier with twice enchanted legendary daedric weapons, a sneak attack only does half damage to a normal dragon, 25% to a Blood dragon, and has virtually no effect on all other dragons.
Although, from the sound of your post it looks like you read my statement wrong. I said it's
more difficult to fight dragons as a stealth character.