Unless they offer a lot of cost savings over the EBRE, I don't see how it could sell (unless they phase out the EBRE or something).

I don't know if the castle folks fully understood or know how to explain the problem, I was never able to make any sense of their proported explanation. The best I can determine it should be fine provided the battery voltage doesn't go below ~6V, causing the switching BEC to drop out, which means the microcontroller in the ESC ceases to function. If that happens, a lot of bad things can follow which lead to catastrophic failure. With two NIMH, you have a lot of leeway before you hit ~6V; the reason this warning came up before was when people were asking about running it on a single NIMH or a 2s LiPo.