We've just started a campaign with an aarakocra monk. He'll eventually have an insane amount of movement speed but, at the moment, he's probably one of the most vulnerable characters.
The first encounter: kobolds, outdoors... including a winged kobold. The aarakocra is immediately engaged in a 1-on-1 duel at 50' altitude where the rest of the party can't really help him. Sure, we can use missile attacks... but he's out of range of touch (or even short-range) spells, and the rest of us are engaged by the regular kobolds (so, firing with disadvantage while in melee). It limited our ability to focus fire on a single target. And then the monk got hit, reducing him to 1hp. That's when the player realized the implications of getting knocked unconscious at 50' up. The falling damage would have killed him instantly. He chose to immediately disengage and land, out of sheer terror at his vulnerability in aerial combat.
From a combat perspective, there are too many ways in which a flying creature can be counter-balanced. They become a focus for aerial or missile-firing attackers. There isn't much cover or concealment in the air. Getting knocked unconscious becomes "probably lethal", rather than "somewhat inconveniencing". Even once the aarakocra reaches a level where a fall won't reduce them to "negative max hp", the damage taken after falling unconscious = 1 death box. Like any situation where one PC is physically separated from the rest of the party, it makes teamwork tactics that much harder. Many enclosed adventuring areas (dungeons, castles, caverns, sewers, etc) simply don't have the vertical space to take the aarakocra out of reach anyway.
Now, there will be times when the aarakocra will absolutely shine - and that's perfectly okay. There's not much a bunch of zombies can do against an aerial archer. But how is this different from having access to the web or spike growth spells (to name just a few)? These spells utterly ruin non-casting, non-missile, ground-based foes. The solution is to have a mix of enemies that can pose a challenge to the party, while also sometimes letting the PC's "win" for their choice. If the party is standing in a long hallway and 4 animated suits of armor appear at the far end, the druid's spike growth is basically an insta-win. If the party is attacked by wolves and the aarakocra sorcerer starts blasting them from above, that's pretty much the same. Good job, enjoy the victory. Hopefully, you've got something that will handle the gargoyles in the next encounter...
To me, the broader risk is what it does to the Exploration pillar of the game, rather than the Combat pillar. However, even there, you've already got to account for the ranger's hawk companion, the wizard's owl familiar, the warlock's imp familiar, etc, etc. There are so many aerial scouting options available that I don't see how the aarakocra substantially unbalances it.