It's funny in the sense that a lot of people wanted to see parasite be used on Luffy. But it's becoming rather clear that parasite doesn't work on people that are strong, even in the case of Jozu I think he wasn't manipulated by Doffy, but merely stopped in his tracks. I guess Doffy will have to pull a Bleach and explain all his abilities to the enemy (and readers) for them to accept this. Viola telling Rebecca that she won't be upset and to close her eyes, man this will translate really great in the anime (with music and all that jazz haha)
It worked on Sanji as well, thing is Luffy can stretch so even if you try to hold his body in place he could just bell you, Luffy could probably outright break the strings by fuussenning and twisting. Also have to consider potential Haoushoku breaking strings like Doffy's presumably broke out of the ice.