Alright, you're right, I shouldn't say NEVER do it, but it shouldn't be done even as often as once in ten matches. Your opponents WILL adapt at that rate, and at 21+ frames of animation, both overheads can actually be VERY easily reacted to, throwing the whole "use it unpredictably" thing out. I mean, I see people react to Claw's throw on startup, and that's only 9f. It's more than twice as fast as Claw's overhead, and nearly 3x as fast as Flame's overhead. The average human being has is able to read and react to an image in 215ms, or 12-13 frames (or 1/5 of a second), so if they get used to seeing that move and don't even have to consider the properties of it after seeing it, then they can instantly react, putting you in an extremely dangerous situation. Someone who's been playing fighting games for 20 years probably has closer to 7-10f reaction time. It means that they could see it and read the specific move twice over before it comes out.
I'll say this though, if you're able to confirm it reliably, then go for it. Claw's overhead is only -2 on block, which just means that you're punishable by only a few characters. If you can't confirm it every time, then it's really not worth it for the meager 270 damage when you're opening yourself up for a free B&B or even HD combo, considering you're at -12 on block. -12 is A LOT in XIII. When you put yourself at -12, Hwa Jai and Takuma, for example, can literally kill you from full health with 1 drive and 1 meter, no HD combo necessary. Most characters can get 500+ for 1+1. On a 50/50 mixup, it's just not worth the risk unless you're going for the kill.
After looking at all the data I may be actually changing my vote for the better overhead to Flame, simply due to the range of it. It makes it impossible to punsh at max range, even for characters with instant grabs.