Fair enough. But even so, has anyone done a breakdown on how many of what CR the spell will affect? I get that you have to whittle down the HP for same CR foes (iirc a moderate challenge?), but at what CR does it just stop being effective without some (or significant) whittling? CR8, CR9, CR10, 11? higher? I mean, HP totals will vary, of course, but it would be nice if it had some effect for those (many) foes a player is likely to face that are unlikely to to be vulnerable without a lot of set up and effort for your one ninth level slot of the day.
The thing is, a level 17 character don't need PWK to deal a hundred damage.
The number of cases where you absolutely must deal exactly 100+ damage are very few. It's not a pro. It's a con.
In 99% of the cases, you simply deal as much damage as you can. Or wait for the next fighter in initiative order.
Under no circumstance is this spell worth your attention.
But it doesn't have to. As I have said before, if you're an CR 12 Archmage fighting L9 heroes, it's very good. Excellent even.
Even high-CR enemy casters can find a use for it. Yes, even if the monster is fighting exactly the level 17 heroes I just said should avoid the spell.
Why? Because monsters in general have much more hit points than heroes.
After even one of your bruisers have managed to get in a solid hit on the party caster, he's toast if you target him with TPK. Getting rid of possibly the game's most dangerous being (a high level player character full spellcaster) is excellent utility.
But let's not get overworked about this. I said "find a use of". Not "excellent".
For the spell to be excellent, and truly worth 1) a level 9 slot and 2) its space in the PHB, you really need to consider using it against PCs that never had a hundred hp in the first place.
This is because of the thing I haven't mentioned, but many of you have: the uncertainty. If you even blow PWK on someone with 101 hp once, it's enough to make the spell suck harder than a <bleep> <bleep> on <bleep>.