My fireball problem (in B/X) is the fact that, in wilderness adventuring, the magic-user (MU) can destroy most encounters with a single fireball.
Facing another MU is deadly: he can cause a TPK with a single fireball.
The fighter will probably survive, but so wounded that the magic-user now has a decent chance of finishing him with a dagger (if the fighter can close in right after the fireball; otherwise, a magic missile could finish the job).
MUs are also very susceptible to fireballs, so if two MUs are fighting, it boils down to "whoever wins initiative wins the duel".
In addition, fireballs are recovered every day - while the HP lost takes days or weeks.
So the magic-user is likely to have his fireball prepared at any time (if you roll for encounters once a day - more than that feels strange to me), but the fighter may be wounded from previous encounters.
I do not think I'm the only one to have this problem - the Rules Cyclopedia limits fireball damage at high levels, and AD&D has complex mechanics to allow for spell interrupting, and monsters with magic resistance.
There are no easy solutions here.
I think old school spells are just not balanced. And the MU is a glass cannon - does lots of damage but is incredibly fragile.
I suggested a system in Alternate Magic which limits fireball damage to 2d6 per spell level. The fireball is just as deadly, but a 10d6 fireball requires the same resources as a fifth level spell (e.g., you must spend your 5th level spell slot).
Come to think of it, maybe spell recovery should take a bit longer (if you're using 1d3 HP recovered per day).
Maybe recover half level per day in spell slots?
So, a 6th-level could always recover at least one fireball per day, but not two, and definitely not all spells at once.
I don't know. Maybe I should just accept that I need to stick to an entire new magic system instead of trying to adapt old school Vancian spells.
P.S.: I have a similar problem with old school dragons and their breath weapons. 1d4 dragons causing 40 damage each... if the PCs lose initiative, they are (literally) toast before they can run. Dragon battles end in the first round unless one of the dragons save successfully. I might change dragon breath to 1d6 per HD to give their victims a better chance.