I tried MND Kaguya, she seems horrible to me. Didn't have enough mnd to shut damage out, didn't have enough HP to take the minor hits, and damage numbers were exceedingly unimpressive.
Yeah, she's definitely not a tank when you build her mind. She's just not going to croak after a single ether flare or whatever. And she should take 0s from the weaker spell nukes like element wind, dancing leaves, etc. And the whole point of using her as a mind build isn't do damage, it's to stone bowl >=P.
As for royal flare+koi3, etc. My point wasn't for those two spells specifically. If you want to treat it on a per spell basis, consider croaking frog, eiki's nuke, starbrow break (though that's a bit of a waste normally since its low delay and flan is fast), Megawatt cannon, etc. Also consider that bowl has a smaller delay time than the barrage as well. I mean I know you probably switch her out after using barrage, but sticking around until you need to switch out has its uses too (I suspect this is why people have a hard time on 18f or whatever, keeping people out who can survive for longer gives you better endurance). Royal flare is however better when your target is weak to fire, or strong to spirit. Or if you have significantly better mag on patchy than you do Kaggy (if you build kaggy for mag it should be pretty comparable but who knows about equipment and skillpoints).
Basically stone bowl has the potential to do everything anybody else can do only better pretty much, except for Yukari's spiriting away. So unless barrage is your absolute best no comparison nuke, or if your only other nukes are too squishy and/or unavailable, stone bowl should easily be the better option provided that you keep it in mind when you're watching action bars.
I don't know why I'ms aying this. I'm not trying to convince people anything here. Just that mnd kaggy has her uses too, and as I've said, I'm pretty sure I'm the only one who does that anyway *shrugs*