A lot of people say they prefer the Zealous Hatred rules over Righteous Fury. I haven't tried the BC rules yet and I'm curious: why is ZH so popular? One of my group's favorite moments in DH and DW is the cheer that goes up around the table when someone rolls a second '10' on RF. Since most of the ZH 1d5 results are Fatigue, that seems a bit anticlimatic to me, compared to the potential massive (sometimes even daemon-killing!) damage from RF. Plus, it seems like a lot of extra bookkeeping for the GM, with lots of Fatigue being accumulated by all of the NPCs. Sell me on it: what am I not getting?
In part, you answered your own question. Having a player roll lucky and one-shot a hive tyrant with a flamer or something might be very satisfying for them but really just "siiiiiiiigh, great, there goes THAT encounter" for you as a GM. Make them fight hard for it, they'll be just as excited and the GM will probably have had some fun with it too. That is the big reason I think ZH was developed - to prevent the anticlimactic Daemon Prince one-shot with a lascannon…or equally likely, pre-errata heavy bolter.
Also for those enemies who can RF, which IIRC by canon is all Master-tier and a number of elites, ZH gives a little…cushion for players. RF makes it that much more likely for the CSM warleader or Ork Boss or whatever to flat-out kill you with extra damage, whereas ZH lets them slug it out a little more, get bloodied up but have a smaller chance of being one-shotted by a lucky roll.
My players want hordes to be able to ZH now, just because they think everyone should be able to get a lucky hit against them. With RF, that's just asking to oneshot a player even more with stacking d10s of damage from horde traits AND RF. With ZH, they can take a licking and ultimately show off and prevail, which keeps them excited, scared of hordes, but not so worried that every fight might kill them,