Regeneration is not the same thing as enhanced recovery. Hisoka is still seen with his arm in a cast presumably weeks after his fight against Kastro when Gon called him up to challenge him, while Meruem after his surgery is immediately ready to fight again. Kurapika only mended broken bones. Pitou is physically regenerating whatever tissues that actually died from the injury as opposed to putting a band-aid on it and let your body heal over weeks. You pay a huge premium for getting things done quickly, and for a good reason. If anybody remotely decent jumped Hisoka even after he had his arm reattached, he's likely totally dead since Machi told him he can't even do vigorous exercise while waiting for his arm to heal so we can safely rule out combat. But if he was healed by Pitou, he'd be immediately ready to fight again after she's done. There's no comparison in how reliable the recovery technique is for anyone who is constantly in danger.