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

Optimization · Pure Optimization
Mapping dungeons and engineering party builds against chartable threats.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.
Etrian Odyssey Untold: The Millennium Girl leans Optimization (Pure Optimization); Fallout: New Vegas leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Etrian Odyssey Untold: The Millennium Girl
Turn-based menus require no dexterity
Fallout: New Vegas
Shooter inputs with VATS assist
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Etrian Odyssey Untold: The Millennium Girl
FOE patterns are scripted and chartable
Fallout: New Vegas
Scripted factions and enemies
Etrian Odyssey Untold: The Millennium Girl leans clearly harder on variance.
Etrian Odyssey Untold: The Millennium Girl
Unknown floors and ambushes punish until mapped
Fallout: New Vegas
Some loot and crit RNG
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Etrian Odyssey Untold: The Millennium Girl
Party builds, skill synergies and map routing dominate
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
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.