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

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

Optimization · Pure Optimization
Scheduling scarce time and resources across a base run by copies of yourself.
Fallout: New Vegas leans Optimization (Pure Optimization); The Alters leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
The Alters
Third-person walking and menus, no twitch demands
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
The Alters
No adaptive opponent, only systems and story beats
The Alters leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
The Alters
Unscouted events and alter reactions surprise first runs
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
The Alters
Time, resource and crew scheduling under deadlines is the 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.