Fist kills everything in its radius, even if its behind a wall or a decent bit above the impact site, heck, even if its above them through a floor. Nova does as well, but as other people pointed out, you can kill yourself with nova bomb, you can't with fist, and Nova takes much longer to fully fire, and fist gives ridiculous damage resistance, even without the perk. I've seen a titan use their fist because I stuck them, and they didn't even kill me cause I was far enough away, completely wasted just to survive a grenade and have me kill them a second later. Melee, fist, shoot them with one bullet from an AR, fist. There are panic nova's, but generally it happens far less than with titans, and a lot of times you can still kill them, either before launch or at least take them with you.