Yet to see any jobs better suited for kill all floors than kraken Gandiva rng. Leader floors it still holds it own. Only floors I'd argue other DD are better suited are Mega Boss floors but even then.. it's marginal.
Also,on anything that doesn't have crazy evasion. kraken rng outdamages normal rangers by a large amount in voidwatch.
Let me preface the following by saying that RNG is my favorite job and I am usually biased
toward it, but not in this case.
First off, any decent gun with last stand is going to destroy Gandiva, especially when you don't have access to full-time potency. So your argument should read, "Yet to see any jobs better suited for kill all floors than annihilator/armageddon RNG. Armageddon might actually be better than annihilator since you have access to wildfire for things like flans/mousse.
Secondly, I have a hard time believing a job without any AoE capacities would be better suited for a kill
all floor than a job like a BLU (AoE spells) or WAR (Fell Cleave).
Thirdly, unless you have enough haste to counteract the 10% slow (not sure if this works exactly like slow), and have enough accuracy such that you can use red curry buns, I can pretty much guarantee that KC RNG will not out-damage a regular pew pew RNG in voidwatch. The constant influx of dusty wings works against KC RNG as well, as you don't need to shoot for TP as often as you would otherwise.
Also of note is that you cannot /war if you are using a KC as double attack will slow down the club considerably (if double attack procs then the 2-8 will not proc). Thus you lose berserk as well. So, you are essentially losing: red curry (150 attack), velocity shot (+15-20% attack with emp body), and berserk (+25% attack). You might as well throw the loss of minuets in here as well considering that you'll probably need march to overcome the velocity shot delay.
To conclude, it's a fun toy, and completely useless. Well, not completely useless I suppose, if you are a diehard RNG and must go RNG to nyzul no matter what. Just make sure you have a BLU for sudden lunge... Like I said, I love RNG, but I wouldn't hold my group back by sticking to personal preference.
Edit: I'm actually willing to admit that armageddon + kc might work well in nyzul since wildfire is not affected by such a major loss of attack. I've never heard of anyone doing this though, but I may have to give it a try sometime. You could mow through the amorphs pretty quick I would think.