u3, I hear, has some whiffing issues, but otherwise works pretty solidly. Also d1 is really good for AA, and leads to more damage overall. He doesn't necessarily need a better d2 since his AA options are mostly fine anyway. The character has really good jump-ins on top of that.
He doesn't need anything, you're right. I just don't like the mentality of only nerfing top characters, because they don't need anything. Kung Lao always had issues with his speed, whether it be movement or pokes, he always had hitbox issues with his teleport, on top of it's speed making it an irrelevant move. Now that he's mid tier or so, those changes would have been really nice.
It's not like i'm suggesting things that would improve him in his current meta anyways. It still wouldn't be good to throw F4 at a downed opponent with 32-35 frames of recovery. The 17 frames of start up, it being a single hitting move, and it still having over 30 frames of recovery, would still make it really hard to read delayed or non-delayed wake ups with. 43 frames is just too much. Kenshi's F2 in Kenjutsu is the same at 40 frames.
U3 has some whiffing issues. The difference is that it has great range, so the whiffing up close kind of keeps the move in check. Otherwise it would essentially be a 9 frame D2 with some of the best range in the game. Since it whiffs up close sometimes, you won't always be able to use it on reaction, like you would with a quick D2. Alien's D4 is the same. Crazy range, that's balanced by whiffing issues up close and heavy negative frames on block.
Alien's D2 has whiffing issues and bad range, which is why I would like hitbox improvements or 7-9 active frames. Wouldn't hurt or help Alien that much, because like you said, he already has good anti airs that lead to full combos. It's just a nice fix to have, to round out the character, especially if he'll be seeing a lot of nerfs.