I think they will balance it after early access. It is hard to say what is viable or not, till it is not playable, unless you are a dnd5 veteran.
In pen and paper setting a failed saving throw and your character is killed, was a bit cruel, so I understand the changes the new dnd books had to make.
There are no OHK spells on 5e if i remember correctly. Finger of death deals only 7d8 + 30 necrotic damage (
https://roll20.net/compendium/dnd5e/Finger%20of%20Death#content )
Keep in mind that on 5e, feeblemind can end the day of a high level caster. Mainly non INT based casters (
https://roll20.net/compendium/dnd5e/Feeblemind#content ) On 3.5e, a edition who had OHK spells had it as a circle 5 spell(
https://www.d20srd.org/srd/spells/feeblemind.htm )
A lot of CRPG adaptations include OHKspells so they don't need to ""balance"" it. On 3.5e, most enemies with high hp also has high FORT saves, so they are very likely to resist FoD. On 2e, most enemies with high hp has to roll low to save vs death. I an playing dark sun wake of the ravager and a encounter that i could't beat by any way become a cakewalk due the dominate(tier 5) spell. The encounter that i an talking about is the mindflayers. They happen to have strong guards who has poor saves vs my DOMINATE
