ExploderWizard
Hero
Then again the 1e Balor was kind of weak too. 8+8 HD was lame.
Pre-UA, the balor was kind of nasty. 75% magic resistance. An 11th level caster had only a 25% chance to affect it with magic. Lower level casters might as well not try. Do you realize how powerful the Symbol spell powers are in AD&D? A 7th level party would be wiped out with stun or sleep symbol.
Of course, a party that just happens to have 4 or 5 double specialized fighters, with girdles of giant strength and magic weapons, will make short work of it, much like a party using feats and magic items will have an easier time against the 5E version.
Any edition requires extra modification by the DM if the PCs are tricked out beyond the baseline assumptions.