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

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

Optimization · Pure Optimization
Faithful X-COM successor managing a global war against alien invasion.
Fallout: New Vegas leans Optimization (Pure Optimization); Xenonauts leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Xenonauts
Turn-based; no physical demands.
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
Xenonauts
Alien AI follows consistent behavior rules.
Xenonauts leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Xenonauts
Hit rolls and unseen alien positions punish certainty.
Xenonauts leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Xenonauts
Air war, economy and squad doctrine decide campaigns.
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.