Witch bolt suffers from a lot of problems.
- At levels 1-4, it is usable... ish... but severely mediocre. Even if you're able to sustain the effect throughout combat, it's outperformed by magic missile. (Remember, the up-front damage of magic missile is supplemented by cantrips on subsequent rounds, while witch bolt costs your action as long as you keep it up.)
- You have to maintain concentration and expend your action every round, costing you the opportunity to cast other leveled spells.
- The target can break the effect of witch bolt by moving more than 30 feet away from you, and the spell does nothing to impede movement.
- Its initial range is only 30 feet. Magic missile is 120.
- It deals lightning damage, which is a moderately-resisted type. Magic missile deals force, which is almost entirely unresisted.
- The upcasting potential is pathetic.
- And all that is just at levels 1-4. As soon as you hit level 5, witch bolt goes from "severely mediocre" to "utterly worthless." Never mind magic missile, you do more damage just by casting fire bolt every round. It's a level 1 spell that's outperformed by cantrips. 'Nuff said.
It's too bad, because I really want to like
witch bolt. I'd love to play a PC who specialized in Force lightning. I'd be willing to take a mild hit to effectiveness for it. But
witch bolt goes way beyond "mild hit to effectiveness" and deep into "self-sabotage."