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.
Nope instakill spells are fantastic and amazing fun. Go and spend 10 years playing dungeons and dragons online to make a viable DC caster - you need 12+ past life feats, 6+ epic past life feats, difficult to obtain gear and then you can finger / wail / mass hold high difficulty content. Otherwise you can still be viable on lower difficulties, but good luck finding low difficulty groups in such an old game where everyone still playing has sunk in the time and effort to make uber characters.
The content can scale to match the difficulty. Ok so if FOD / Wail fail its a wasted spell? This is also addressed in DDO - on a save against wail enemies still take negative levels if they are susceptible to level drain. On a fail against FoD, it becomes the second most powerful negative damage spell second to necrotic ray. Based on the game being an MMO with hit point inflation a saved FoD can still deal over 1500 nectrotic damage on a correctly specced pale master, necrotic ray can crit for over 3000, which doesnt matter much when normal enemies eventually scale to over 15k HP and bosses to over 100k.
But the DC spells work and are incredibly fun to use. So do damage specced sorcs - draconic sorcerers can hit for like 50k damage with their specialised elements, but good luck with fire which happens to be the most resisted element in the game, though this is also manageable on a fire specced sorc by also having some acid or electric spells on the side as well.
Character building is where the fun is with DnD - a level cap of 12 doesn't really offer much character building options.
Some finger of death / wail of the banshee footage here, bearing in mind this is also a 14 year old video and just in a basic explorer area of the game:
Oh a 12 year old video of my own of farming some epic level mobs in the first 5% of a raid for whatever it was that they dropped, epic scrolls or something:
Wail / FOD / Circle of Death / Flesh to Stone / Mass hold / and much level draining to get them to work utilized, it might not look great but it was incredible fun to play.
Eh also pudding summoning rogue / monk of some kind cos why not:
