Question: I just read John's version of the Human Clay Boon and it broke my brain. A god can use it to give a Hero/Demigod free Attribute increases up to 5's in all nine Attributes. Is that right?
Actually, a god can use it to give a Hero or Demigod free Attribute increases up to 5 in eight Attributes, and up to 6 in the ninth. But yes, basically that's how it works, and how it's supposed to work.
The unspoken subtext in your question is the idea that this is broken because it might allow PCs to get free dots in attributes without paying for them with XP, but we're not worried about it at all. Lemme explain:
First of all, we're not worried about NPCs using this all over the landscape, and you shouldn't be, either. Sure, Apollo could always Human Clay his Scions up to their maxes in Attributes, but he's not going to. For one thing, that's 10 Legend he needs to use for his own issues, thank you very much, and for fighting the Titan war that he and the rest of the gods are busy trying not to get creamed in; giving Johnny Scion three extra dice to his Attributes is not going to win out against being able to bust off Bleach twice more in a moment of crisis. For another thing, the first thing every god visiting their Scion in the World is not going to do is spend 10 Legend and get Fatebound to everything in their vicinity (in fact, I'd be very shocked if most gods didn't come down in an Avatar too low-Legend to use Human Clay anyway). The entire point of Scions is for the gods to not have to spend a lot more Legend on small stuff or get a lot more Fatebonds; if Human Clay were inexpensive, then sure, they'd probably be using it to buff their newest soldiers, but as it is it's unlikely that anybody wants to spend a twelfth of their entire Legend pool on a bonus that, to them, must seem incredibly miniscule.
Those are plenty of in-game reasons that Storytellers shouldn't be having gods just buff Scions with Human Clay at random; out of game, it should also be pretty obvious that that's not the power's intent, so don't use it as a Storyteller fiat to just hand out extra bonus dots unless you have a good reason in the plot to do so.
But PCs can also get Human Clay! Yes, they can, and that's good and we like it. They're being run by players, so if they want to blow a significant portion of their Legend pool on buffing their own Scions, Fatebound mortals or lesser immortals they hang out with, that's a choice we're not going to stop them from making. Because they're not getting Human Clay until they're gods anyway, there's very little chance they'll be able to use it to "cheat" extra dots for the rest of the band, but if they happen to go up to Legend 9 and still have a PC friend who's Legend 8, we wouldn't stop them from doing that, either. For one thing, again, it's a significant Legend investment for the PC doing it; I'm here to tell you that even when it's a matter of "seriously, I might die, oh god help me", players are not always willing to blow all their Legend on their fellow players when they might need it themselves, and that is only magnified when it comes to ephemeral things like "but I could get my next Epic faster!". But, more importantly, it also probably doesn't matter very much.
And it doesn't matter very much because of Fatebonds. By the time one player can use Human Clay on another to try to give him maxed out Attributes, both of them will be heavily loaded down with Fatebonds, and the Fatebonds will simply say no. Nope, you don't get to have five in every Attribute, because you're negatively Fatebound to three, and they'll simply be bought off again; and if you were positively Fatebound to it, you've already got at least five dots in it by Legend 8 and Human Clay can't help you try to get better faster. Nobody is going to be able to suddenly shoot up to huge numbers of Epic Attributes on the strength of this boon even in the rare cases that it can be used player-to-player (and the player in question is willing to blow his own in-game resources on someone else's out-of-game bonus). The only Attributes that this can really help all that much are those with no Fatebonds attached, and at Legend 8 our PCs usually had an average of one of those each.
Of course, it is a slight XP bump for the player who has it used on them by another player; even if a negative Fatebond buys the dots off, it'll redistribute those points elsewhere according to his or her positive Fatebonds. But we've talked about our lack of concern over XP disparaties before, and in any case, this is again something that is unlikely to happen except in extremely rare cases.
And we do want it to be something that the boon does, because Human Clay is very much a specialty boon that doesn't see a lot of use, and we want it to be useful and cool enough for Scions not to groan when they buy it. Situations in which you sculpt other lower-Legend creatures or beings into new forms are comparatively rare in the game (though Scions who really want to do it can of course go find people to perform it on), and while it's a power that should definitely be available to gods of Health, most of the time nobody wants an expensive high-level boon that lets them pay an expensive Legend fee to cause purely cosmetic results in their targets. It needs to have some mechanical benefit attached, and the Attribute bonuses provide that in a way that is concrete enough to matter but not major enough to break anything.
Incidentally, we've actually never had anyone use Human Clay in-game to increase or decrease a target's Attributes; it's been used a few times, but in each of those cases was popped in order to take advantage of the ability to monkey around with the target's bloodline and genetics.