I would argue that Jozu was doomed against Aoikiji, though. It seemed that Kuzan was in his trademark chill state when he got him. I don't remember there being a single hint of a technique Jozu could do to block that. I would even argue armament Haki would not work well. It probably would help, but Jozu was indeed basically one-shotted by Aoikiji, and I do not believe any form of distraction on that battlefield, at least, would have altered that outcome.
However, that is not because I consider Jozu weaker, so I am not disagreeing with you, M3J. My reasoning is that this is a specific case of a type disadvantage. No matter how hard, a diamond can still be frozen, because that is a state it cannot affect or resist. This is one reason why Aoikiji is probably one of the scariest close-combat specialists. He can totally halt and kill targets and even slow them down from low temperature. Unless they can combust or produce significant and continuous heat without effort like magma and fire and heat, there is no direct or easy counter to Iiiice Time.
Now, to bolster your point, there is the case of Akainu. Jozu is diamond, and diamonds are tempered at not only extreme temperatures, but pressure as well. I would personally believe that Jozu could even be a natural counter to the magu magu no mi. Notice that Akainu stays well away from Jozu. So the Commander could be a real threat to at least Akainu, and quite possibly be worse to Kizaru, given that a diamond could easily act as a prism and scatter Kizaru to oblivion, but also because even if he cannot directly harm Akainu through fruit nature exploitation, he probably is totally resistant to magma punches. I would be amazed if a fire or magma punch would hurt a diamond fruit user. It would be like Luffy taking an normal fist. Knuckles won't work. And if Jozu has CoA, then, well... it would take a long time for him to go down.
So yeah, the Yonkou Commanders, at least from the WBP are no joke. But I do honestly believe that they just weren't as cool as BMP's. Or honestly as powerful in sheer war potential. There's no telling about Marco's power level, since he was cuffed through most of the fight and did not get to go wild until the end, when there was no longer a point.
There's also this fact to consider that I only now thought of: it was an actual battle, a war. Cracker, for instance, would be glorious on the battlefield. I would want his power on my side before Marco's or Vista. Because an endless legion of warriors with incredible Armament and natural defensive power is something to fear. Whereas, Jozu's power was just not designed for battlefields where an enemy force has multiple options of dealing with someone of his type. In a one-on-one, I firmly believe he actually had a fruit suited to combat or at least survive solo against 2 out of 3 Admirals.
But in a war, that 1vs.1 advantage meant 0. The vulnerable Admirals did not bother with personal pride and did their duty where they could serve best, and the one admiral who is an instant win button against Jozu confronted and pwned him.
It makes me wish we had the BMP as a whole in Wano, and not just Smoothie. Because it would be interesting to see just how their Commanders fair in an actual battlefield. Cracker should be top tier. Katakuri though? Smoothie? I don't know.
So yeah, power level wise, the Commanders are up there. They can surely fight against Admirals and hope to at least not have to flee for their lives immediately or die, like the Supernova did at Soambody, but power level is just raw strength calculation. In terms of true overall "power" there is strategy and combat type involved as well. And that's where I think the WBP got nerfed. They were just not suited for a war of that scale.
Makes me wonder why Marco didn't just swoop down carrying Vista on his back and try to snatch Ace while Whitebeard sank Marinford. Because goodness knows he should have been able to do that much. . . but then that would be too easy and no fun for the readers.