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

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

Optimization · Pure Optimization
Engineer a pandemic; evolution order is the puzzle.
Fallout: New Vegas leans Optimization (Pure Optimization); Plague Inc: Evolved leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Plague Inc: Evolved
no reflex
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
Plague Inc: Evolved
world response scripted
Plague Inc: Evolved leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Plague Inc: Evolved
country event timing
Plague Inc: Evolved leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Plague Inc: Evolved
evolution order heavily solved
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.