head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; King's Bounty: Dark Side demands more variance.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.

Optimization · Pure Optimization
A turn-based tactical RPG about army building from a demon's perspective.
Fallout: New Vegas leans Optimization (Pure Optimization); King's Bounty: Dark Side leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
King's Bounty: Dark Side
turn-based, no manual execution needed
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
King's Bounty: Dark Side
scripted AI armies, no true adaptation
King's Bounty: Dark Side leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
King's Bounty: Dark Side
combat rolls and loot randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
King's Bounty: Dark Side
army composition and spell-build depth
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.