yea, neji isnt winning this one. hmm now that i think about it neji might be able to notice the chakra heart thing sasoir has. dont know if he would be able to destroy it though.
would be a really good fight imo. sasori's needles, and posion smoke I think could be countered by rotation and byakugun's field of vision. And as far as the 100 puppets goes neji could also counter that with 128 palms (see how fast he was with the spiders). And i do think that neji could see sasori's weak point, heart container. I feel like one good shot with air palm could prove fatal for sasori.
i don't think sasori is weak nor would this would be easy for neji but i don't see how neji couldn't handle sasori's techs. As long as he can counter the 3rd kazekage he should be ok.
though I must say neji does take a lot of them down, but either he gets scratched, or his stamina gives out. Sakura was able to wreck a few of these puppets, so I see neji with his vision and reaction speed doing a lot more.
I think it's also important to remember that Neji would have no problem cutting the chakra strings to the puppets and Neji will notice right away that the only part of Sasori's body that emits chakra is a container in his chest...the more I think about this fight Neji should be just fine.