Piccolo pisses himself and cowers in fear of powers (SS Goten, SS Trunks, Kaioshin) much weaker than SS Goku. At the Cell Games, he got the shit kicked out of him by someone about half as strong as the weakest adult Buu arc Super Saiyan (Gohan), and nothing implied he powered up a lot since then. Kaio-ken Piccolo would still be basically useless compared to Mr. Buu, SS3 Goku, SS3 Gotenks, and Ultimate Gohan.But yeah, Kaio-ken Piccolo would be absurdly powerful. Considering that he's already not that far off from Cell Games Vegeta and Trunks at that point in the series, he'd easily be able to pass the Saiyans and even Cell himself if he just powered up using a few levels of Kaio-ken, let alone x10 or x20! Considering that SSj3 isn't even a 10x boost over regular Super Saiyan, and Piccolo isn't that far beyond the regular Super Saiyans... Piccolo could easily surpass SSj3 Goku if he could use high-level Kaio-ken ontop of his Buu saga power.
Perhaps Buu's gum-like body just makes him ill-suited to the Kaio-ken. Otherwise, there's really no reason that he couldn't just copy the Kaio-ken from Goku upon seeing it used a couple of times and roflstomp everyone.And Buu if he could learn it, or Uub after reaching his potential, would be insane. Fat Buu Kaio-ken x20 vs. Kid Buu would be a really odd (...and anti-climactic, considering just how much stronger Fatty would be with that kind of boost!) alternate ending to the Buu saga.
It's never said that Kaio can't use it (indeed, he'd have to use it just to confirm that it... you know, exists). It's just said that he couldn't MASTER it.I'm guessing Kaio-ken isn't quite compatible with godly ki, though--would help explain why the North Kaio could never use the technique he developed, since all of the gods should have at least some god ki even if their ki isn't 100% god ki like Beerus and Whis
Goku was still more or less fine after using the highest Kaio-ken levels against Vegeta and Freeza. Yeah, he was out of power, but he wasn't dead or anything. If, say, Mr. Buu could use it, it wouldn't actually matter if he was tired afterwards. Because a simple Kaio-ken x2 would let him one-shot Pure Buu. Same goes for Kamiccolo and the androids. Heck, if they could teach Kibitoshin Kaio-ken, he'd probably stomp all over Pure Buu.It probably took too much of a toll on the body. Just look at Goku using it against Vegeta and Freeza. Sure he was getting a bit of an upper hand on them but at the end he was worn and torn tired. Those were the times Goku got pretty beat up. I can imagine him sharing that to the others. Maybe Tenshinhan and Yamcha didn't wanna use it cause of their "pride". Doubt that but maybe.
As I said, I don't think this explanation really fits. But if one REALLY wants to go the "they need a strong body, not just a high amount of ki" route, they can assume that Goku's comment about being able to use higher level Kaio-ken because he got stronger was referring to his body physically toughening up due to the x100 gravity he endured. That'd basically mean that the five zenkais were for raising his base power level, while the gravity training was allow for the usage of Kaio-ken x10. This is supported a bit by the fact that Goku still can only use Kaio-ken x10 comfortably both before and after his last zenkai, despite the enormous difference between his pre and post zenkai selves in battle power/ki.I don't see a reason how it can be so taxing on the body when at certain levels it Kaio-Ken noticeable. At 90,000 x2 and x3 pretty much feel like nothing to Goku. They don't strain nearly as bad as before. At 3,000,000 x10 is what Goku can do without straining himself. Now someone with a massively higher power level such as Piccolo later on should not have a problem using the technique since he's way stronger than Namek Goku. Cell is much much stronger and should logically know of the technique since the spy bot was around. Yet never uses it. He could have demolished Gohan as an SSJ2 if he used a x10 which didn't feel like much to a 3,000,000 power level Goku. Oob would be monstrous with Kaio-Ken with his high power. I only have the god ki stuff as a maybe though as perhaps it doesn't work with godly ki, though really it probably should work.
I can't see how someone much stronger than Namek Goku can't wield Kaio-Ken. Even teaching wise I don't see how none of them could learn that technique. If we go by DBO and Xenoverse, all races can use Kaio-Ken.
Chapter: 270 (DBZ 76), P11.1-4
Context: Goku has turned the gravity back down to 1g.
Goku: “Hwoo!!! I…I feel so light… …It’s like I’m not even here!!! Hoo hoo hoo!!! I’m light, I’m light! And strong, strong! Like this, I can definitely withstand about a 10-fold Kaio-Ken!”
