I've been using the frenzy spells the entire game and they only ever consistently fail against city guards (due to the way in witch they level), as long as you keep putting perks into things like kindred mage and rage you should be fine, also there are 3 different levels of frenzy (Fury - Novice -, Frenzy - Adept -, Mayhem - Master ) which one are you using?, what level are you? and what illusion perks do you have?
Good tip ... I've been using Fury all the time and failing, which is a real pain on a sneaking character. Looks like I'll need a trip to Winterhold to see if they have the new level in stock ... I've just got Illusion to level 50 for Quiet Casting, but the spells have been slow in coming into the teachers.
Also, Courage is a great way to level Illusion because you can cast it repeatedly on friendly NPCs and it costs very little mana.
Calm is great in the wilderness if you don't want to have to fight bears and cats. You get some funny comments from humanoids as well when you Calm them. (e.g. "Well, that spell didn't do anything!")