<< In a frost prone area [RdB] will most likely need temporary frost protection in the fall . . . >>
<< Side by side with RdB, Etna types are much hardier >>
Well, that's just not consistent with my experience. We are certainly frost-prone here, beginning in October. I provide NO fall frost protection. I manage the trees as if adapting to progressively colder temperatures is a necessary part of the tree's process of preparing for winter.
Meanwhile, I treat my in-ground RdB exactly the same as my two in-ground Mt Etnas, Hardy Chicago and Marseilles Black. For example, this fall I covered all of them in early December after many days in the low 30's and some into the mid/high 20's. Roughly the same the prior two winters. And RdB and MB performed comparably -- zero damage either year. HC suffered roughly 30% top damage last winter but more like 10% the year before.
But I did protect them. I have no data or opinion on relative performance stark naked.
I agree with your comments on vulnerability of trees when actively growing. In the fall, my trees are not actively growing (and I am careful not to fertilizer after mid-summer); so growth then has never been an issue. [I do suspect, however, that slow transition to dormancy could sometimes be an issue for some other varieties. But even my potted plants, including tender varieties such as Smith, endured multiple frosts before I put them in the garage this year, and they all seem just fine.] Then in the spring, my protection has been adequate to discourage growth until I remove that protection in early/mid April. The plants don't actually leaf out until ~May 1. In four years, I've never had damage to growing plants in spring.
I would also agree with a thought that is perhaps implicit in your comments -- poor protection is a risk. In particular, if the "protection" actually warms the trees (e.g., by creating a mini-greenhouse) it could encourage early emergence and increase vulnerability to subsequent cold. But so far, I don't seem to have that problem.
It is possible that RdB is slightly quicker to emerge from dormancy / quiescence than the Mt Etnas and therefore may be more susceptible to the whipsaw of warm / cold cycles in early spring. So RdB may be more sensitive to the quality of protection. I just haven't observed a difference here.