I remember when playing ad&d, phantasmal killer was a great spell for its level. In BG3 it is not worth it unfortunately. In fact I didn't really use spells that could be negated with a saving throw. You risk wasting your turn completely which can be catastrophically for a spell caster. Most fights you just had to deal out as much damage as possible as early as possible in order to win, because of the big advantage of offense vs defence.
