Robin was threaten by her crew's life and the trauma from her past (Sanji's case: Zeff and trauma/past is Vinsmoke). Both life threatens because of the individual requires them (World Government:Big Mom). Opponents are considered great powers for their standards against the crew at the time (CP9/Word Government, Buster Call, Aokjii: Big Mom, Vinsmoke). They both gave in to their threatener's deal and are leaving crew because they want to protect them. And then, Luffy comes to the rescue despite the immeasurable odds against him and the protest of the one he's trying to rescue (even it be easier to abandon them).
and you mentioned how Luffy isn't even aware of the threat on Zeff's Life and Sanji's cuffs. The same applied to Robin, where the crew initially weren't aware of why she wouldn't return after their first rescue attempt failed (the same thing with Sanji's case: first rescue attempt failed while not aware of the threats being used against him when they tried rescuing him on first try).
See the similarity? That why I said KINDA being HYPOCRITICAL considering Sanji's involvement, smarts, and determination he showed during Robin's case (and the faith involved in it all).
Also, I'm not blaming Sanji. I said I be disappointed in him if he continues this Robin like path and development.