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

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

Optimization · Pure Optimization
A grand strategy 4X where empire optimization carries you through random crises.
Fallout: New Vegas leans Optimization (Pure Optimization); Stellaris leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Stellaris
Real-time pausable with no input precision.
Stellaris leans clearly harder on reads.
Fallout: New Vegas
Scripted factions and enemies
Stellaris
AI is scripted; multiplayer adds modest adaptation.
Stellaris leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Stellaris
Galaxy gen, anomalies, and crises vary widely.
Stellaris leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Stellaris
Empire builds, fleet comps, and economy dominate.
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.