I was reading a reddit post earlier, and someone was saying that rage actually currently stacks with backstab, so I thought maybe they reworked how "backstab" is defined, maybe instead of 100% crit, it's now "x2 damage", sort of like how sneak attack works in skyrim, gets a damage multiplier but not actually crits.
I feels like this approach is better than the plain backstab = 100% crit, as rogues naturally gets crit chance from wits and other sources, and the crit chance would be useless if backstab functions in such a way.
I cannot confirm this at the moment, but others may feel free to try it when they have free time.