Poison Daggers are Bio type damage. Few units have any substantial resistance, this is usually balanced out by most of these type of weapons being relatively low damage.
The dagger however has 50% scaling with 2 stats and 50% armor pen. With both stats at max you get 130 base dmg with a 50-150% dmg roll, half armor, low cost so can move alot or swing multiple times. And that not even counting potential armor stat boosts. Also reasonably early on the tech tree and reasonably easy materials to get.
Downside there are some units, usually mechanical, that are fully immune, and not gonna get any captures with it.
Snip
(shot, not thrown = can't throw at all inside the base because of the ceiling)
snip
Not entirely true, one can crouch and get some throws off. Certainly limiting but not impossible.