Well most of it was said already, but in defense for Jack a little powerscale:
Jack is someone that pretty much suffers from the fact, that Oda is great at his own powerscaling and pretty consistent with it, cause he's always keeping the balance.
Jack is a gigantic fishman at base, basically like a Wotan, just because he's a big fish type of fishman, matching the two handed swings of guys like the Dukes and Ashura Doji with just one hand, who in turn are already physical beasts. He got those top notch genetics already and on top of it he has a Mammoth fruit, that gives him great durability, enhanced endurance and enhanced physical strength.
If Oda would have given him a fully developed Nitoryu fighting style he wouldn't be a 3rd Commander anymore, because he would then end up becoming far too powerful. I expected him to be less skilled, but I also wouldn't have expected him to get no named attacks from Oda. Oda definitely did him dirty, but nonetheless he's still very strong.
He would simply bulldoze through guys on and below Yc3, if they are not super lethal or destructive for their tier like swordsmen or Ace. And he's basically a one man army even against very strong fodder. He's like one of the top beasts of Rusukaina with a conscious mind. 1bill for that monster is totally justified.
Above him are only guys with very powerful DFs, max skill fighting styles, adv Haki types and op racial features, so..