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

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

Optimization · Pure Optimization
Combat-focused Infinity Engine RPG built on D&D party optimization.
Fallout: New Vegas leans Optimization (Pure Optimization); Icewind Dale leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Icewind Dale
Pausable real-time needs little dexterity
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
Icewind Dale
Enemies follow fixed AI scripts
Icewind Dale leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Icewind Dale
D&D dice rolls and random loot tables
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
Icewind Dale
Party builds, spell prep, and encounter knowledge
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.