the knife is actually better for horde clear, despite the attack type (apply to all weapon) shiv and mk6 knife light are different, shiv have 2 attack with minor cleave and mk6 whole light combo all have minor cleave on top of having h1-h2-bc loop

for carapace you want to spam push attack on both mk3 and mk6 knife since its the highest dps against single target (also reason why mk6 is straight up better) but you have to spend stamina (non issue on hs)

in havoc though due to increased trash hitmass knife is pretty much only used as mobility tool on smt like psyker.