head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; The Witness demands more optimization.

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

Optimization · Pure Optimization
A puzzle island where learning each panel's hidden language solves everything.
Fallout: New Vegas leans Optimization (Pure Optimization); The Witness leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
The Witness
No reflex demands; line drawing only.
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
The Witness
No opponent; island is static.
Fallout: New Vegas leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
The Witness
Puzzles deterministic with no randomness.
The Witness leans clearly harder on optimization.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
The Witness
Decoding the rule systems IS the whole game.
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.