I tried getting this to work with Snake for an hour. Apparently I just can't do it. :/
Have you tried changing your controls to accommodate for DACUS? One common way you can do this is setting your L, R, or Z button to Attack...as that gives you one more frame in order to time your DACUS. Snake's DACUS is still relatively easy to do, although I do believe that the timing is slightly stricter than it was in regular Brawl (due to a Jump Start reduction most likely). Also if you have problems with immediately doing your DACUS, you can still perform a DACUS after running for a little bit. Just make sure to remember these tips:
1) It is usually better to rotate the control stick Up after mashing C-Stick down.
2) Rotating the control stick Up, as opposed to just mashing Up, yields more results.
3) Make sure that a VERY BRIEF amount of time is in between mashing C-Stick down, and mashing attack. Since this sometimes causes errors with a character's DACUS coming out.
4) Despite what note #3 says, DACUS smashes must be performed with split-second timing (even when using Attack instead of Grab). It takes a pretty long time to get adjusted to for each character, and it is still easy to make mistakes with them due to their split-second nature. DACUS smashes have to be performed at a bullet-like, yet steady and fluid pace in order to work.
My recommendation is to try DACUS smashing with these characters first, as they "feel" as though they have the easiest DACUS smashes to perform:
1) Jigglypuff
2) Lucas
3) ROB
4) Ganondorf, Ike
5) Falco, Snake
6) Wario, Link
7) Peach, Squirtle
8) G&W, Wolf, Roy, Marth
The list above is just something I quickly put together, and is subject to change. But yeah, despite what the frame data is for specific characters, some characters just feel naturally easier to perforrm their DACUS smashes with. I assume it has to do with how quick said character's dash attack comes out (plus other factors). Either way though, I would say to start practicing DACUS with Jigglypuff, and then work your way down until you can perform it with the other characters. I won't blame you if you can't do it with characters in #8 though, because those characters still give me trouble as well, lol.
If you still need help, I'll do the best I can to assist you by going into training mode with Snake, and writing a more detailed response with my findings. I hope this helps!
On a separate note, I think if Sheik got her DACUS back it would a really good thing
This would be both good and bad. Good because she gets the furthest sliding DACUS in the game back, as well as one of the strongest DACUS smashes in the game. Though it would also be bad because her Jump Start would have to increase from 3 to 4...meaning that her jumping animations comes out a frame later. This would mess up people's timings with her aerials, SHFFLs, as well as just the general fluidity and swiftness that is associated with Sheik. Sure, it is a very small change, though I would prefer Sheik to have the fluidity that she had in Melee as opposed to her having a DACUS.
Not to mention her DACUS was one of the meanest in the game...and because she's already a really solid character, she doesn't really need it. lol. She still has Boost Grabbing shenanigans though, which is a very nice alternative.