One of the problms in Oblivion is that you're Illusion spells were rendered useles once got past level 25, because the NPC's would then be to high a level to be affected.

Has this problem been fixed in Skyrim?

In Oblivion you could create your own spells that worked at any level. In Skyrim, you need to make sure you're using the best spells and spending perks in the illusion tree if you want your spells to land. Do that, plus make sure you dual cast the spell to make it more powerful.

No, in the spell creator in Oblivion you only had the option to choose 25 as a magnitude for Illusion spells, which means only creatures and NPCs up to level 25 would be affected by them.

No, thats not how I remembered it. I had a Breton Vampire mage that could land mind control spells even at level 60 on most creatures in the game. Not to mention the fact that the mana cost was so low that by the time I finished casting the spell I would be at full mana again.

Not saying you're wrong because its been awhile for me since playing it, but I do know that my illusion spells never failed when I cast them on stuff that wasn't naturally immune to them, NEVER. Even at level 60.

I have a Master Illusionist in Skyrim. Check out my video where I play with Giants, mammoths, and Pacify a Draugr Scourge Lord.

I will note the Master spells are useless and impossible to get. I will say that the spells in Skyrim are garbage compared to the crazy stuff in Oblivion. Dual Casting is a must for Illusion.

http://www.youtube.com/watch?v=mal3o9AEr9M&feature=g-upl