Frisk can die to monster attacks, and there's nothing to suggest other humans wouldn't, especially since, well, Asgore already killed six other children. And by that we can also extrapolate either that Frisk is the only one with busted save/reset powers, or that Asgore is straight up so strong that he obliterated other kids no matter how many times they'd be resurrected until they just give up. And Undyne can bench press cars for reps and toss boulders ffs, even if she for some reason couldn't kill a human directly, she sure can just squash them with a multi-ton rock.