head to head
Neither is simply harder - they are hard in different places. Endless Space 2 demands more reads; Fallout: New Vegas demands more execution.

Optimization · Pure Optimization
Space 4X with politics and faction-specific economies.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.
Endless Space 2 leans Optimization (Pure Optimization); Fallout: New Vegas leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Endless Space 2
turn-based
Fallout: New Vegas
Shooter inputs with VATS assist
Endless Space 2 leans clearly harder on reads.
Endless Space 2
AI scripted, some MP
Fallout: New Vegas
Scripted factions and enemies
Endless Space 2 leans clearly harder on variance.
Endless Space 2
system traits, anomalies
Fallout: New Vegas
Some loot and crit RNG
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Endless Space 2
economy, ships, populations solvable
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
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.