How come Cell never used it?
Buu has a limit to his regen right? Fat Buu nearly got killed by Kid Buu. So he could overdo it if it was too high a multiplier or he did it for too long I suppose. How much damage KK would do to his body per second idk - his body is strong asf, but is that enough for his body to not take big damage from a high kaioken? Or does his body need to be trained in a specific way to handle kaioken? Like, when Goku was training on the way to Namek, did he learn to handle KKx20 just by becoming stronger or did he have to work on his body's tolerance to kaioken separately?