I spent ages working on a Conjuration based build, the idea being that with a high enough Conjuration skill I could get Daedric armour from the Winterhold shrine/forge thing which with a Breton's magic resistance + the lord stone would make me nearly invincible.
But I need a full set of ebony armour to make it AND the boots don't work
And screw illusion, there's nothing you can do with it that a potion can't do equally well, restoration too, even if wards block more damage a resist-whatever potion is a equally valid defence and it doesn’t use up your mana. Furthermore you can make conjuration staves at winterhold so you can piss all over learning that skill, the only reason anyone learns alteration is for the stoneflesh spell and mage armour is just a liability at higher levels, why bother with it when a set of heavy armour + a fortify heavy armour potion does the same thing, except without wasting your mana.
And all these work-arounds are simply because the game discourages you from levelling up.
But you will anyway, I mean who doesn’t try to unlock the chests?
You know there's goodies inside, but lockpicking raises the lockpicking skill, making the game harder, but it's not a combat skill, so it
just makes the game harder

Ditto for speechcraft, it's not like you can calm a bandit and talk your way out of a fight.