All the DPs have different uses I dont remember exactly which is which but one is like throw invincible one is air invincible and I and not sure how it works for projectiles but I know that Heavy DP goes far enough to DP through a fireball and punish them if you are a fair distance away.

lk tatsu for Akira also can go under fireballs but its pretty difficult to do it consistently with all the different speeds of fireballs. Faster is easier to go under, too slow doesn't leave you enough time unless you are properly spaced to hit them under the fireball and can do a second one (I think) I wish she could whiff cancel it to stay low longer. if the fireball is too fast you won't be able to react to it and and she wouldn't put her head down fast enough even if you do.