About Uriel VI the one kind of injury that I can imagine could prove irremediable would be brain damage. I see boosting INT as entirely different from brain surgery or treatment of psychiatric conditions ... If your brain is damaged through asphyxiation or impact such that your thoughts are fled then healing the physical aspects are not going to return those thought patterns ... then you have to consider the link with spirit. So in Uriel's case I would imagine that his spirit fled and there was no way to return it short of necromancy.
Boosting intelligence, hm. Well, what is "boosting intelligence?" Rather, how does it work? It cannot simply make you smarter, especially considering typically you only have a time limit on the spell's effect.
So, I put on an enchanted necklace that boosts my INT by 10, as long as I wear it. What is it boosting exactly? Possibly, it stimulates functions of your brain to use at an elevated level. They say, to which I disagree with, personally, humans use 10% of their brain. Perhaps boosting INT forces it to use more. But that isn't relative to intelligence. In fact, we're more or less born with a maximum potential of intelligence that is different for every individual. You can raise your IQ, but that itself isn't relative to intelligence. This goes deep into the anatomy of TES which I am simply boggled over, yet interested, because it is fascinating. But there isn't enough information, I think, about it. How do the brains of those in TES work? Do they have brains? I honestly do not know, it's not information I ever attempted to learn, nor am I by any means a lore expert. I know they have bones, hearts, etc, obviously. But I do not recall anything about brains.
I want to type a lot more, but among stress, lack of sleep, relationship problems, and being WAY too excited about conversations like this... my mind is really racing and I am losing my grip on forming coherent thoughts.