Just like the rest of the Straw Hats, Sanji's will is indeed immense. The problem here is, as usual, how he's compared to Zoro instead of understanding the character alone and measuring him from there. At the end of the day, not only they are deeply different but Sanji's context and background is way more nuanced and troubled: the moment Sanji grows up on wanting to repay Zeff their debt while Zoro grows up on wanting to fulfill his promise to Kuina, the moment they obviously can't be measured on the same terms; it's not coincidence that Zoro gave up on his ambitions and was willing to die for someone as soon as he actually had someone to care about (Luffy and his friends) who he valued as more important than himself (just like Sanji towards Zeff).
Most of Sanji critics simply want him to be, think and behave exactly like Zoro because they can't see beyond that character archetype, basically (either that or something more pathetic, like wanting to be like Zoro or seeing him as a role model).
By the way, for those who bring up the Fishman Island scene: Luffy himself showed doubts on whether finding One Piece was more important than finding a rare beetle.