In illusion I see all these spells that only effect targets of a certain level or lower. Perks can modify this but the target levels still aren't that high. Won't I just end up fighting things too high level for my illusions and have a pointless character?

Because that would be really dumb....

You'll be able to affect everyone with the right perks and dual casting.

Haven't worked the numbers here. Does it require a huge amount of investment such that it would require being an all-out illusionist? Or could it be viable to play an Illusionist/Assassin? Like... would having to invest in sneak, 1-H, light armor, etc make it too difficult to be competent at illusion later in the game?

There are three keys to ensuring that Illusion remains viable regardless of your level: (1) the perks on the right side of the tree; (2) dual casting; and (3) bringing the cost of Illusion spells down to 0 through Enchanting.

(1) These perks are important for successfully casting Illusion spells against higher level targets. Kindred Mage and Rage, for instance, combine to increase the maximum effective target level of Frenzy against humans by 18 levels.
(2) Dual casting effectively doubles the maximum effective target level of your spell.
(3) Unfortunately, dual casting costs 280% of the spell's base mana cost. At 100 Illusion skill, that means Mayhem still costs 1636 Magicka when dual cast. Thus, you need an Enchanting Skill of 100, 8 Enchanting perks (5/5 Enchanter, Insightful Enchanter, Corpus Enchanter, and Extra Effect) to put 4 x Fortify Illusion (25 pts) on your gear for free Illusion casts.

Illusion is incredibly powerful, but if you skip any of those steps, your spells will start getting resisted more and more frequently as you level up, until they become ultimately useless.

Sounds like I'd have to go all-out illusion then to keep my character relevant. That's lame...

Getting 100 illusion skill is no biggie though. Cast muffle for 30 mins. and you're there.
Enchanting is easy too. Craft some iron daggers, buy lesser soul gems from the winterhold academy teachers, enchant banish/absorb health = make loads of money, skill smithing as bonus and get Enchanting 100 in 2-3 hours.

With smithing + enchanting up, you're unbeatable anyways. And with the alchemy glitch you can 1-hit everyone...

Okay none of that stuff sounds fun at all. Do you guys really do that? Spend 3 hours of your lives pressing the same button?

It sounds like you're approaching Skyrim, like a power-grind MMO instead of an single-player RPG.

No I#ve just read about the tricks and glitches.

I play a sneak archer atm with no illusion/enchanting/smithing etc. skill whatsoever.

It's more fun when you're not invincible.

Illusion is basically an all or nothing skill.
If you don't spend perks on it, it's rather useless lategame.
If you do spend perks on it, it's awesome.

I wasn't trying to talk you out of Illusion. Every "pure" Mage absolutely needs either Conjuration or Illusion for crowd control. Conjuration doesn't require dual casting like Illusion does, so the Fortify enchants aren't quite as mandatory, but you'll still want them (along with Destruction of course). Conjuration spells are plenty expensive, and needing to resummon your Dremora Lords in battle without free casting can result in a dead Mage pretty fast.