Vulns are only a problem if they are being exploited.

Backstab damage/bonus affects/edges are very str dependant so duergar/svirf have a fair advantage for them. Human is good too as with 20 str they have access to the edges as well.

Not sure if knife cares about str but both skills care about dam roll and there is a lot of higher end dam gear for duergars other races can't match in many gear slots.