The last time the Ainur wielded that kind of magic, during the aptly named War of Wrath at the end of the First Age, entire continents were rendered uninhabitab

The last time the Ainur wielded that kind of magic, during the aptly named War of Wrath at the end of the First Age, entire continents were rendered uninhabitable and sunk below the sea. It might or might not have been an admonishment about the unprecedented destructive power of nukes. The Istari were sent to lead and inspire the peoples of Middle Earth, and being clad in flesh was supposed to give them greater empathy for their plight. By leading with force they likely would not have been able to resist the temptation of the Rings of Power ane would have turned into dark lords themselves, like what happened with Saruman. They could still access their full power, but were only supposed to use it in extreme emergencies. The Balrog of Moria was unaccounted for in the plans of the Ainur, and likely would have wrecked Gandalf's plans completely even if the Fellowship had managed to escape it.