I guess this is why the half-casters in the various editions always seem to be clerical or druidical? Fighter/magic-user is just too powerful a build, from the BECMI D&D elf to the modern hexblade problems you mention.
One of the things that always got me about powergaming, though...if half of these things were real, people really would be exploiting the wazoo out of them. Wizards would haste armies, flaming spheres would be cast into the enemy's granary, lords would send clerics to cast contagion on the enemy's troops...
Don't hate the player, hate the game.
That's why as a DM I really only ban classes and multiclasses that don'tmatch the setting, over or under power the base assumption of the game by a lot, or obviousl take up too much spotlight.
Because if D&D were real, 40-60% of adventurers would be powergamed and munckined to hell.
80% of the humans.
And I'm lowballing.