The weapon has assault, so it's worth pursuing.
As for Rahab herself, her summon effect is +thunder resistance. So, great when the event is thunder, pointless effect outside of that. As for the passive... it seems to be a case of, if your dps is largely from direct damage skills (ie, you're using the Spartacus/Achilles/Hercules line), then Rahab's a priority for water users. Otherwise, just stick with Jormungandr or Garuda.