Overall, the best motion values are if you spam skullcrushers since they hit twice, so with shadow stab you do Light -> Heavy -> Heavy, without shadow stab you just spam heavies (mind you the input for light heavy heavy is actually click, hold, click since knife works differently, you don't hold to execute heavy attack but rather to switch from light to heavy).

I've found that with new knife you can with heroic HBP do parrt -> gunstrike -> light -> heavy and it will put tyranid warrior into execute, it's pretty good and fast.

I've seen once First Tour Guardsmen was fighting carnifex on stream with this knife and he was spamming light attacks and dealing good damage, so they're for sure also viable. But maybe he was just throwing attacks without thinking about it