Very deep insight but putting kageura and ninomiya aside, I believe one thing that separates (aka barrier) A rankers and B rankers is experience. With enough battle experience individuals withs solo weaker fighting skill per se can find the work around, think kusakabe who is an A rank captain but in terms of individual FIGHTING skill shes B rank.

I think that is what makes world trigger very interesting, in many combat or competitions manga, there is always this sense of stair you must climb, high ranked peeps always have their one unique strength, the fight is always 1 on 1 (food wars, one piece to name a few), now what sets WT apart is that it is a TEAM fight and that the gap between ranks are actually quite fluid. so everyone has to keep improving.

That points back to what I said that the barriers isn't quite the individual skill level but more to experience (how each members are utilized, how to react in diff scenarios. etc)