OHKOs were stupid and not fun. Reloading a fight until you OHKO the big guy is not a good game design. I'm glad they got rid of it ine 5E.
1 - The game is not designed around save scumming. You need to maximize the save DC to have a good powerful OHK spell
2 - You can't reload in P&P
3 - You can't reload in multi player NWN1/2 games.
4 - Enemies casts this spells on the player too.
Better than single summon limit but still not like P&P.
Madscientist explained well how things works on the game.