Honestly, I think this is more sensible than looking at total power.
I do that for solo content, but for raids, it's hard to (V)MVP with Joan (actually it's just hard to (V)MVP in general, but even harder with Joan...) And helping out the raid with Joan/Andromeda is sort of a thankless job >.>Chances are if you need survival, you're better off using Joan instead of more base HP, no?
For dark ult, all DoT cap out at 750, so high hp actually really helps. If you're increasing your in-battle hp from 6k to 7k though, dun bother, it just makes it harder for you to heal up as Slashley stated. I went from 10k to 15k by messing around with eidolons and FLB-ing one of weapons, and it helped a lot for dark ult.
Ok, maybe when I have time I'll take a look.
Actually they kinda are - the slope of the line suggests that you have to give up about 3.5 atk to gain 1 hp, which is why high hp event eidolons always look so awful in total power. This is also why I caveat everything with if you need the hp. I actually often find that trade to be a bargain for AAB builds where the AI does a lot of stupid things so I need a bigger hp pool to offset its stupidity (I AAB AQ5 and most ults - not dark ult, actually I AAB that, too and rely on friends to clean up my mess ) If you don't find the hp useful, do what Slashley does and only look at atk (in which case Tiamat rules, by a pretty big margin.)I'd always had the impression that event eidos with high hp got shafted on attack, but seems like that isn't necessarily the case.