Well the beta is for testing the spec, so if Harbinger are underperforming they will fix it.
I'm planning to test the spec in fractal with Berserker, Viper, Grieving and Diviner (and maybe Marauder too just to see if the vitality could help). As for pvp, I don't think this will be a good pvp spec but I can forsee a Harbinger with Eternal life trait to not be that squishy...
We will see next tuesday..