I'm only thinking it cause I was splitting wood today.
Dull doesn't matter actually, sharp makes things worse when they stick. Piercing is easy, it's retracting that's the trouble if you impale something deeply especially if they're basically liquid sacks. That's why stabbing weapons for the military have to have blood grooves so you have some air and you can pull them out. If you're thinking he just cuts down once he's impaled someone shearing them in half (which I've never seen him do after stabbing someone) there's still a substantial amount of suction created once impaled that's hard to get around. Or to put it in easier terms, take the sharpest samuari sword in creation that doesn't contain a blood groove, stab it into four inches of wood then try pulling it out. If you can do it in less than thirty seconds without wiggling it or putting your foot to it you're freaking hercules.