head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Optimization · Pure Optimization
An open-world shooter-RPG won by perk routing and weapon mod optimization.

Optimization · Pure Optimization
A pirate-themed action RPG sequel trading Gothic gloom for muskets, voodoo, and tropical islands.
Fallout 4 leans Optimization (Pure Optimization); Risen 2: Dark Waters leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 4
FPS aiming, VATS softens demand
Risen 2: Dark Waters
Clunky melee with dirty-trick interrupts; modest demands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 4
Scripted enemy AI
Risen 2: Dark Waters
Scripted enemy behavior throughout
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 4
Loot and legendary rolls
Risen 2: Dark Waters
Light loot randomness only
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 4
Perks, settlement, weapon mod planning
Risen 2: Dark Waters
Skill investment, voodoo builds, and quest order decide power
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.