head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Hearts of Iron IV demands more reads.

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

Optimization · Pure Optimization
WWII grand strategy: templates, production, and focus paths.
Fallout: New Vegas leans Optimization (Pure Optimization); Hearts of Iron IV leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Hearts of Iron IV
pausable, micromanagement-heavy
Hearts of Iron IV leans clearly harder on reads.
Fallout: New Vegas
Scripted factions and enemies
Hearts of Iron IV
PvP partly, mostly scripted AI
Hearts of Iron IV leans a little harder on variance, though the gap is small.
Fallout: New Vegas
Some loot and crit RNG
Hearts of Iron IV
focus tree branches, events
Hearts of Iron IV leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Hearts of Iron IV
division templates and production 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.