Assasin isnt a special job because Vinera is part of the story. Nothing about Fryevia screams "Im Exclusive", if you want, you do you man. My lvl 99 whisper as spellblade subjob does well anyway.
Vinera is defintely a good UR unit if you need a dark slash unit, I think shes a good contender against WoL and Engelbert. Vinera, Kitone and Lynx is a dream evasion PvP team tho...
there will be an event when you need dark slash unit, raid or something i guess if you look at JP.
Initial reaction might be that combination of jobs is underwhelming, but depending on her stats, AI behavior and what skills/spells she's gonna get with those jobs might just make her very powerful.
Only problem is that Gumi decided that she's a limited time unit, so many will skip her and save for WoL that is proven in JP to be powerful unit.