It had a thief class that really wasn't good at combat, and Vancian casting to make the 5 minute workday possible. Moldvay 1981 I believe didn't have BECMI's weapon mastery system, so caster dominance came into play past name level, an no I don't consider followers to make up for that. I could go on.
For the record, I don't entirely agree with DMKastmaria's post. Or, rather I don't disagree, but I don't think "thinking beyond the character sheet" is a good argument for saying a game is balanced.
OTOH, I don't think the above are good examples of B/X being imbalanced. The thief is not bad at combat. They have low hp and high AC, so they're not good at melee, but they get Cleric to-hit tables and their natural high Dex gives them missile fire bonuses. Also, at 10th Level they can use magic scrolls. They're good light artillery. Plus, they make up for their relative combat weakness with their skills. No, it's not 4e type balance, but it's a kind of balance.
An argument can be made about the five minute workday, but that's a bugaboo of every edition of D&D, even
4e. It's just sound tactics in any case to go into an engagement fully charged. But still, it's not a big problem because of the way the casters are designed. Because they have weaker spells than most other editions, and fewer of them, there's less of an ability to nova in such a way that the party with a caster out of spells greatly cuts down on the party's viability. Lower hit points and swingier damage also discourage excessive combat engagement, while reaction rolls and morale checks can cut short or even avoid combat altogether. Sure, that's a particular playstyle that not everyone likes, but it's balanced.
Caster dominance is not a problem at all in B/X unless the DM really
loves casters, and gives them every spell, and plenty access to magic scrolls, wands, and other options, basically houseruling it into 3e. The highest level in B/X is 14. At Level 14, casters get 2 sixth level spells. That's it. And the majority of spells in B/X are utility spells. The big guns, Death Ray and Disintegrate, are not automatic game winners. Death Ray is essentially a minion killer, and doesn't even work on creatures with 8 HD or levels (read: anything likely to be a good fight for Expert level characters). Disintegrate works on one creature, but they make a Save vs. Death Ray. Virtually all monsters use Fighter saves, the best saves in the game (save for demi-humans), and Death Ray is the Fighter's best save. Polymorph Self gives you the physical abilities of what you turn into, but not any special abilities (like breath weapons) and you keep your normal AC, hp, and to-hit rolls. Cloudkill is another minion killer, doing a mere 1 point of damage to any creature with 5 or more hp, and is tough to control, liable to being blown by the wind or sinking down holes or hills.
Really, some of the more powerful spells are the ones like animate dead, animate object, conjure elemental, or invisible stalker. And all these do is basically give you low-level temporary retainers, like the fighter has. Meanwhile, the fighter's also got the best saving throws, the best AC, and best to-hit, and due to the non-uniform XP tables, is usually a level or two ahead of the wizard.
Because they use similar conceptual mechanics, B/X and BECMI typically get lumped in with 1e, 2e, and 3e. But I think the level of balance they achieved is underestimated. They didn't fall victim to the constant powering up of casters in the other editions, with more and more spells, more spell slots, and even more powerful versions. I think WotC's R&D could get a lot from really looking at B/X and BECMI, so I'm glad to hear Mearls' is trying stuff out using a B/X game as a springboard.
