Eh i think it´s fine.
Being able to whack a large bunch of non CW elites out (no shields) very quickly seems overpowered for a second but then you realize that there are a ton of things in the game that do that. For instance Waystalker trueflight can in a vacuum of other enemies easily demolish 2-3 stormvermin, with a conc pot one can be relaxed and still decimate one of their patrols even if they all carry shields.
As for other enemies?
I personally think Unending hunt is generally much better for anti horde&dealing with just a few elites, particularly so when getting multiple waves of them and it might even be better for dealing with chaos patrols. Bosses?..Admittedly fervency does the best against bosses but on the other hand its still headshot reliant, not too long and not even too powerful when dealing with those.
If i am not mistaken, fervency also removes the teamwide crit buff when using the ability right? Having the team frequently get 25% bonus crit has a pretty tangible impact, that and he can still crit on his ranged attacks with unending hunt.
So…nah i do not think there is a problem with it, its neither overpowered nor overwhelmingly superior.