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

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

Optimization · Pure Optimization
Optimizing pair-up chains and offspring builds to steamroll deterministically scripted maps.
Fallout: New Vegas leans Optimization (Pure Optimization); Fire Emblem Fates leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Fire Emblem Fates
Turn-based menu play; no hand-speed demands
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
Fire Emblem Fates
AI follows scripted attack priorities, not player modeling
Fire Emblem Fates leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Fire Emblem Fates
Dual-strike/guard procs and hit rates inject meaningful RNG
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
Fire Emblem Fates
Offspring seals, asset/flaw selection, and supports are deeply planned
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.