Alright, so, I just posted this in the Black Crusade Rules Question, but since the Talent appears identical in Only War, I'm going to cross-link it here.
Furious Assault Talent - What's the trade-off?
As someone mentions in the BC forum, it just seems like an upgrade to All-out attack.
Well, you pay for it with XPs, so that seems fair enough.
Yes, this is pretty much exactly the same as it was in DH. Although there it works like swift attach, the bit about it spending a reaction seems to be just a second check to ensure you aren't using a reaction as well if you have some kind of talent that allows parrying or dodging with all out attacks or other additional attacks that use up the reaction.
I guess that it is there to limit how many additional attacks you can make, as well as making it still a valid option if you were to get Step Aside or Wall of Steel.
If you have 2 Reactions to spend, you could potentially get 3 Attacks with some rather nice bonuses here.