Insta-kill spells were never a good mechanic in older D&D. Let's not pretend having that much RNG is a good thing in any system.
Disintegrate doing 75 or Finger of Death doing 62 damage (+reanimate) in 5e is essentially the same thing, except balanced. They're still really powerful. High level characters and boss enemies have some form of protection at least. And it's cool you have to whittle down a boss a little bit before those spells go "online".
What messes up BG3 is the Long Rest spam and how Larian have beefed up the encounters to fight full strength parties. Inflated stats, saving throws and HP make heavy hitting spells feel weak. Gamey mechanics like Unstoppable on regular enemies make everything feel lackluster. And Larian's hate for control effects has reduced all spellcasters into brainless Magic Missile and Fireball machines.
