Running low on flux energy in the middle of a battle was no joke as it usually meant getting slaughtered by the enemy. In a sense, it was even worse than being physically exhausted since hunters would experience painful cramps and their heads would ache so bad most of them would throw up. In short, it was the single worst experience most hunters could go for, but for Uriel things were different.
Having experienced the pain of death on many occasions without the sweet relief of his life ending made him resistant to the physical pain caused by flux exhaustion. On the mental side, if being deprived of flux energy was bad, the opposite was a thousand times worse.
Back on Chichén Itzá when Water warlock shared his flux energy with him, he felt like a water balloon filled with too much water. Understandably, this feeling far surpassed the mental numbness and dizziness that came with flux deprivation.
