head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Pillars of Eternity demands more variance.

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

Optimization · Pure Optimization
An original RPG system where engagement optimization and passive-stacking theory dominate hard-mode encounters.
Fallout: New Vegas leans Optimization (Pure Optimization); Pillars of Eternity leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Pillars of Eternity
Real-time-with-pause removes all twitch pressure from the player
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout: New Vegas
Scripted factions and enemies
Pillars of Eternity
All AI opponents use fixed scripted behaviour trees
Pillars of Eternity leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Pillars of Eternity
Hidden enemy immunities and ability distributions require investigation
Pillars of Eternity leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Pillars of Eternity
Per-encounter ability management and passive stacking define trial-of-iron mastery
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.