I've tried using Destruction a my primary (not only) skill without the use of exploits and regularly got my ass handed to me (I had to abandon that strategy). When I run out of magicka and am left running for my life, my opponent literally has an endless supply of it. I've noticed this with shouts as well...ever notice how those draugr can spam the hell out of Fus Ro Da, and yet you can't? Yeah, it's pretty annoying, and pretty unbalanced, but then Bethesda aren't exactly well known for their balanced gameplay.
I have heard other people say that they believe enemies have endless supply of magicka, so I paid close attention to how enemy magic users fight. In my observations, I have found that enemies have finite magicka reserve. For example, take a magic using draugr, who summons a frost atronach and shoots frost spells at you. If you have a drawn out fights with him, you will see him summoning the first frost atronach, and after you kill the first one, he will summon the second one, after waiting for his magicka to regenerate enough. And after you kill the second one, it will take him a long time until he summons a third one. If you fight mages or dragon priests, sometimes they seem to have infinite magicka and casting fireballs nonstop. High level mages do have quite high magicka pool, but also they carry around powerful staffs. For example, an Arch Pyromage (EncWarlockFire07BossHighElfM) would have 659 health, 576 magicka and is guaranteed to carry a fireball staff. And he has ward absorb and is pretty good at using ward. I once had a very drawn out fight against one of the dragon priest. The fight was so long that he eventually ran out of charge on his fireball staff. Once that happened, his rate of attack dropped dramatically and was much easier to kill.
About Fus Ro Dah, they do seem to have significantly shorter cooldown than us. But Draugr Deathlords are pretty manageable despite that.