Yamato is too strong. She will mess up the crew dynamics if she joins. She probably is stronger than Luffy
Yeah, this question has always been in my mind but it's just too much now. We've had strong characters join the crew before, but Oda never made it so undeniably clear that they were stronger than the M3.
Robin was OP, but she was very easily dispatched by Crocodile
Franky was strong but he fought against Fukuro whilst Zoro and Sanji fought Kaku and Jabra
Jinbe was a warlord, but he only joined after Zoro and Sanji both got power ups in Wano and now he fought Who's Who whilst they're fighting calamities
Whereas Yamao is straight up fighting a serious hybrid Kaido in a 1 v 1. He's one of, if not the strongest opponent in the entire world. And she's doing this after Oda showed us Zoro losing even with the aid of other supernovas.
Zoro could become stronger than her by the end of the arc after his fight, but there's no chance in hell Sanji is going to become stronger.