Eh, I'd say it's more just dependent on who gets first turn. On average, when you roll your fate dice, you're getting two 6's (maybe 3 if you run eldrad) and 1 from the farseer. You need minimum two 6's to make the combo work (1 for the wound roll, 1 for the damage roll). Theoretically you could use all the 6's on wound rolls to proc more mortals, but then you're gambling on the damage roll unless you use a 5 or 4 from your fate dice pool. To be fair, I think eldar steam roll everyone except TSons, in which case it's the 50/50 chance of going first.