The only disadvantage to a knife is, it ain't that good against certain bosses (especially bonemass since you shouldn't have a sliver one at that time). Otherwise, the alternative attack is great (+ interesting/unique mechanic) and normal attacks have same or better dps than most other weapons out of the same material/biome. Black metal knife for example is absolutely on par with blackmetal sword, and you can kill singular or spread out trash mobs often a lot faster cuz it swings super fast. And if you can land a sneak alternative attack on an unsuspecting foe it does a really big amount of damage (like 80% of lox hp gone with black knife and decent skill in knifes).