Even aside from being worthless against marines and fearless (which is a massive liability in and of itself), I still dislike Soulfright weapons because:
a) Multiple hits don't stack (thereby cripplng any plans I might have of concentrating fire), and
b) I don't know what damage (if any) the unit sustained until it's too late to assign more. I don't want to hit a unit with Soulfright and then ignore it, only for it to double-1 it's Ld test and take no damage. Likewise, if I soulfright a unit and then whittle it to almost nothing with other shooting, only for it to fail its Ld by a ton, I'm going to feel that I've wasted a lot of energy.
Soulfright would be a nice rule to have as a bonus (similar to Soulfire) on weapons that are effective anyway. But, when Soulfright is the only effect of said weapons... no. Just no.