Pretty much half the hardmode magic arsenal, really. It's absolutely dreadful.
Ultimately, magic weapons fall into one of these categories to remain relevant:
- Pierces walls
- Fires from above
- Rapid-fire forward projectile
- Homing forward projectile
- Forward projectile
Later weapons tend to combine these properties and obsolete the ones that come before them, but then when you get into the actual categories, there's barely any progression because either they deal too little damage (Crystal Vileshard, Wasp Gun), cost too much mana to be sustainable when autofired for sustained single-target casting (Unholy Trident, Venom Staff, Nebula), appear too late to really be useful anymore (Dungeon in general, Medusa Head, Toxic Flask)...
So ultimately what you do is this; first craft a Meteor Staff, then fish for a Crystal Serpent, and then you use that until Razorblade Typhoon, which you use to get a Blizzard Staff to replace your Meteor Staff.
Many of the magic weapons that could be good are primarily held back by their mana costs rather than their usage mechanics; the reason that Meteor Staff and Crystal Serpent are so powerful is that they barely cost any mana and they can cast nearly indefinitely when under the effects of a Mana Regeneration Potion without triggering the debilitating Mana Sickness debuff.
Razorblade Typhoon stands out above Nebula weapons because despite its mana cost of 16 per use, each use fires two projectiles that linger for a while and rapidly tear through endless swarms of monsters, making it highly efficient. With Mana Regeneration active, it is also capable of casting indefinitely with nearly no loss in effective mobbing DPS against small targets or even larger targets if you wish to use it on them due to the mechanics of piercing damage. Nebula Arcanum is too slow and costly to allow for a similar use, even if it is significantly more powerful in its actual element since Luminite seems to be its own type of piercing damage without a proper piercing damage cap. Prism and Lunar Flare are powerful and Flare is actually sustainable under these rules, but you've beaten the game at that point and there's nothing to use it on outside of rematches, invasions, and modded content that'll replace it within ten microseconds anyway.
tl;dr let casters have some variety
also nerf holy arrows so the ammunition you use is more of a choice