uair and bair have small, quick hitboxes and require a lot of accuracy. you can't expect to be dead-on each time. dair has a very long duration, so full jump dair, essentially, will keep you safe. shffl dairs can set up grabs or taps or tilts if they're not cc'd. be weary of smart ccers or people with fast reflexes who will cc... cc is the bane of your dair. but other than that, it can be a great and damaging move.
you might not think so because of its horrible range... but remember that the move lasts a LONG TIME, and coming down on the opponent, it's your ONLY option to attack. otherwise, you either have to time a fair (also hard to hit with) or do your best to dodge an attack from beneath, as if you were peach with a parasol. people don't expect dair, and they never will. it's fast and long-lasting.. in fact, one of the longest lasting moves in the game. it's as if you have a shield around your body that taps people for a little bit of damage.
dair also beats things like samus' missile, and does plenty of damage. against fox? fox has VERY TIGHT space control, but dair seems to outprioritize most of his approaches (except for running up and up smashing.) it clashes with nair and both get hit, it stops grabs, and prevents drillshines. you might not see why, but imagine fox coming on top of you with a full jump, ready to drill shine. if you see him and full jump dair, you will be tapped by the his dair, and he will be tapped by yours... but you know what's gonna happen next, he didn't expect the move. it's just like knowing what to do after a cc.
but maybe i looked too deeply into it.. never think too technically. just know that the move is viable because of its duration.
.
