Jinbei has an elemental advantage over Cracker's biscuits and the strength to put him down once past. Also if Luffy taught us anything durability and stamina go a long way in a fight.
Durability n Stamina doesn't last long. Example Luffy ran out of juice at Dressrosa. If it wasn't for plot Doffy would have won that battle.
Its clear as day n night, if you don't have offensive skills then there's no chance u can beat your opponent.
In luffy's case he had KKG to ko Doffy. He had speed n power to beat those heavenly strings.
This things are missing in Fishman Karate or whatever Jinbei showed so far. Not to forget if Jinbei is commander lvl then that completely destroys the dynamics of SH crew(Monster Trio).
Saying someone can't beat Doffy one on one doesn't prohibit them from being commander level when Doffy himself is. Doffy locked up Jozu, broke free from one of Aokiji's attacks, took close to if not the most and biggest hits from G4 before being put down. To think Doffy is anything under 1st commander is weird.
He only got Jozu when he was fighting other guys. His strings cant do anything against his diamond skin. It was clear that only way to beat Jozu was when he was distracted(as seen when Aokiji froze his arm).
And luffy having harder time beating Cracker itself shows that Doffy at best is 4th commander lvl not first.
As for Boa there's a lot of advantages to get past but I agree if you can you have a good chance of beating her but a lot are going to be turned to stone before they can get that chance even commanders I would think. Plus the way Smoker said "Kuja haki" and them being one of, if not, the first to show it on a projectile leads me to believe they may have some above normal haki.
Even BB loves Women does that mean she can turn him to stone???
you can go n watch 3D2Y to see what she can do.