Dreadnautilus5000🌳
Plantera
When you're underground, crowd control is not a priority. What you want are ways to eliminate enemies that pose a threat as easily as possible, and the terrain severely limits the usefulness of line of sight weapons in that regard, no matter the DPS. Since in higher difficulties enemies get much higher stat increases than bosses, being in an enemy's line of sight is pretty dangerous, especially if they fire projectiles. Crystal darts bounce, which allows them to kill enemies without putting you at risk, pierce or bounce between enemies, which can help against groups of enemies, smart-bounce, which means that they'll rarely miss, are fired at a fast rate with the Dart Pistol and deal higher damage with the Dart Rifle. Also running out of ammo isn't an issue while it is a bit annoying, because Crystal Shards are everywhere in the Underground Hallow, and don't forget that ammo consumption isn't factored into rank in this tier list.The thing is that I brought up Underground & Dungeon as my main examples of crowd control, while you brought up Pirate invasion (which is a smaller and least important part arguably).
Blade Staff can achieve just about the same, except it's automatic, you don't need to worry about ammo, the damage is higher against single target enemies and they'll stop and stunlock blue armored bones in place, while crystal darts may struggle to stop an incoming fighter AI on your face.
Also, Blade Staff can only attack one enemy at a time, which means that unless the enemies are in an extremely tight clump, Crystal Darts are better for multiple enemies.