typically dismemberment in any form except from bullets is unrealistic especially in one blow in real life that simply would not happen the human body is tougher than it looks. Even a decapitation would take a huge amount of force on the proper spot, i just dont think limbs flying everywhere is realistic sure more blood but only where it should be never gore for gores sake
I watched an episode of Deadliest Warrior. I believe it was Samurai VS Viking....anyway the Katana sliced through 2 full pig carcasses, at the same time. When I say sliced, I mean completely in half, both of them, through spine and ribcage. I'm sure if I was motivated enough I could find the video. I would however like to point out, that was on bare flesh....when it was tested against the vikings chain mail, it did almost no damage at all. Really a cool episode. Anyways, my point being that if a sword can slice through 2 pigs at the same time, surely it could easily lop off an unprotected arm, or whatever appendage that got hit. Making dismemberment a possible, but unlikely outcome (would happen from time to time).
As far as bullets go, I don't have any experience with military grade ammunition, but I have hunted a lot. I've seen a lot of gunshot wounds made by guns that are larger caliber than most standard military ammunition. I have not one time seen a limb completely blown off, or a head blown off, Even though appendages or headshots are not a common place for a hunter to shoot at, I have seen gunshots in those areas. I do know, however, that most military ammunition has a full metal jacket on the projectile. Which is made more for piercing, unlike hunting ammo, which is more for breaking apart/bullet mushrooming on impact to inflict maximum damage on an unarmored target. I'm not saying it has never happened, but it would seem that it is more likely that bladed weapons probably dismember more often than bullets.