Time to elaborate: D-Air is NOT GOOD (or at least as much as it's made out to be)

[Miku]

Idk i hardly ever use his Uair. most of the time its easily dodged. Everyone sees it coming, im never able to connect it. I connect the Dair like 30 times more often then i do the U air. If ur gonna use any sort of aerial to hit people who are above u. The Bair is a much better choice. They tend to dodge it less, just because there its harder to tell when ur going to use it.

A tip that may help you connect the Uair more often; Go into the controls for your name and turn off tap jump. May or may not be your problem, but I find that not having to even worry about accidentally doing a mid-air jump makes use of Uair simpler and easier to connect.
Additionally, you could try using the C-stick to Uair, leaving your DI control seperate. Sometimes makes things simpler. A lot of people c-stick by default, I don't, but occasionally I use it when I feel like it for one reason or another.

 

[Sosuke]

I don't even use this move. Or Bair. =(
Yup I'm weird. But yeah anyway to the topic. Dair, to me, should barely be used. The only reason people use it so much is because they think the fast falling speed will make it hard for opponents to read, but it has a lot of startup lag so yeah

 

[gkrackerr]

D-air, as stated, is mad situational.

But idk, there are definitely some good tricks for the dair. Like, something I do is full hop, pull a bomb, and just kinda float over my opponent and then z-drop the bomb. Usually my opponent will shield and then in that second of shielding I dair and catch the bomb again. Simply one situation where it's good.

But also, idk if anyone said this, sh-dair out of shield isn't too shabby. Easy to see if your repetitive w/ it, but earlier in the set/match sh-dair after they attack your shield because you will either a)land a hit, or b) poke their shield and open it up for bombs and arrows.

my .02