AD&D has some front-loaded demihumans. From dwarven saving throws to elven attack bonuses (and infravision, among other useful abilities), non-human characters enjoy substantial advantages up front. Humans get the full range of classes with no level limits, but nothing more. Add multi-classing, and demihumans have undeniable advantages. Casting spells wearing plate armor is nothing less than the best of both worlds and a powerful combo humans lack and won't make up for until advancement most won't reach...
So how do humans balance the ledger? Multi-classers divide hit points, so a character with 8 fighter hits and 2 magic-user points would get 5 (8+2/2), which is almost half those fighter hits, and well within a longsword's d8 damage. Add slower level advancement and humanity sees faster progression. But this is apples and oranges, especially considering those non-human abilities. AD&D's solution, and all of early D&D, was to impose level limits, which may come up short because in old-school gaming, low(er) levels are the only sure thing.
Which has inspired Robyn and I, as game designers, to give human characters their own racial abilities*, or in the case of race as class, counterbalancing abilities, with unlimited advancement (where applicable) or identical limits otherwise. Now none of this is to say that AD&D got it wrong. Far from it. But B/X and BECMI, its close cousins, embraced race as class and missed an opportunity to better achieve balance; and given the fun these games continue to offer, level limits are a trivial thing, but maybe one worth reexamining.
*Basic Fantasy, a personal favorite, has been doing this for years...