Also... terrain, also rearguards, point guard, understanding ambush, corners, elevation (what GM lets the fireball magically emanate after the the hillcrest?), initiative is HUGE for the perfect fireball... so, is initiative such that your enemies march in a perfect blobbed formation somehow in unison? Is your party cool with sacrificing their own turns to stay out of the thick of battle so they don't take your fireball to the kisser?
It is not even about specialized enemy knowledge. Its the mix between tactics, terrain features, and turn order... which is a huge factor deciding on if the opponent spreads out or not.
In my game, even in open environment, fireball does prove to be pretty circumstantial and underwhelming compared to other options.
I do run a lot of mounted humanoids trying to hijack things from the party. Also, some pretty windy tombs.. so maybe that is unique?