I play a pure Mage on Master difficulty which is, unfortunately, about the most difficult I can make the game. The game is easy. Many of the skills and perks are extremely strong and the AI is extremely stupid. About the only risk in the entire game is a stream or AOE attack in tight quarters where they cannot easily be avoided. Everything else, from Dragons to Draugh to Giants can be kited and dodged by anyone with a modicum of skill.
With Conjuration most of the game becomes downright trivial. Soul Trap and a corpse puts you at Expert Conjuration for a few hundred gold and a little time. A bit more gold and you've got both Summon Flame Atronach and Summon Frost Atronach, so those aforementioned scenarios that might become difficult are once again trivial as either of those will make short work of anything up to at least level 30. All this is easily accomplished by the time you return to Whiterun with minimal effort, in other words at the beginning of the game.
The game is easy, even as a pure Mage. However Destruction is still terrible.
Destruction spells are weak and do not scale. If you focus on it and pick up the perks it does for about the first 30 levels because you can pick up new spells, grow your Magicka pool and pick up better equipment. That's the point where it stops, and even up to this point it is inferior to everything other than unarmed. Yes, the other magic schools have some powerful spells, but I can just as easily use those while shooting arrows as I can while shooting Firebolts, they are not relevant the discussion of Destruction. After 30 it becomes pathetic, the character becomes weaker while other skills become more powerful.
The problem at it's core is two things.
First, the spells cannot be upgraded, other than a couple perks there is nothing the player can do to improve them and these perks are inferior to those available in other skills. What would help this, and what should have been done, is if the entire Destruction tree had perks devoted to improving all of Destruction rather than a single line of spells. The result of this decision is that each line has basically four perks, and two of the spell lines are all but useless. Meanwhile Archery, for example, has virtually it's entire tree improving it, whereas in Destruction a Mage using Fireball has a good chunk of his tree being completely worthless.
Second, there is a serious lack of potions and enchantments to improve the damage and help it scale. While other skills allow the use of poisons and other potions, Destruction is severely limited. While other skills allow enchantments on both the armor and the weapon itself, potentially greatly improving their attack, the Mage can only enchant their clothing and need better enchantments to improve their damage. If there were a third point, it would be the lack of spellcrafting, but while I cannot understand why Bethesda refuses to reimplement what was a truly innovative offering in Morrowind, it's not strictly necessary to bring Destruction up to par.
All that said, I don't really care. The game is so ridiculously easy it's irrelevant. I'm relying on modifications to bring the difficulty up and introduce spells, potions and enchantments to help magic become what it should be. I've never played a Bethesda game that I enjoyed without modifications, Skyrim is no exception but it's probably closest so they deserve credit for that.