I've been trying to explain this shit since Scires got it started and apparently I still gotta say it again.
Importing the data from LGPE works, but not completely. There's a lot of stuff that still needs to be done on a per-pokemon basis to make sure everything comes in and isn't broken.
Biggest thing is the animations. There need to be new ones added to the file for entering and exiting the pokeball. Scires explained how the game kept locking up on those because they were missing. You could fill them with empty animations, but the point is you need to put SOMETHING there.
Then with the textures, in order for them to not look weird you need more than just what you can import from LGPE and a lot more for the 3DS mon. S/S mon have normal, specular and I think even incandescence maps. Point is, they got more maps that make the textures look more like whatever they're meant to be (ice/metal is shiny, Trubbish's bag is plastic-y, etc.).
For both of these, you have a major issue of the data format that has to be solved, which I believe the person in the OP said they started working on. We need something that can totally decompile and RECOMPILE the GF format the games use in order to add these textures and animations. I know there's things to extract models, textures, etc. but as far as I know there's no compiler yet.
After that you also gotta worry about reintroducing and rebalancing the old pokemon. A lot of their stats can carry over simply enough, but things like abilities that aren't programmed in will be much harder to figure out and likely need to just be replaced. Same goes for adjusting move sets. This'll likely lead to a lot of different groups trying to make their own balance patches with the reimported mon that'll be all over the place.
Just getting them in is already gonna take time. Things are certainly on their way, but don't expect a full National Dex reimplementation that doesn't look or play like shit for at minimum 6 months.