head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more optimization; Risen 2: Dark Waters demands more variance.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.

Optimization · Pure Optimization
A pirate-themed action RPG sequel trading Gothic gloom for muskets, voodoo, and tropical islands.
Portal 2 leans Optimization (Pure Optimization); Risen 2: Dark Waters leans Optimization (Pure Optimization).
Risen 2: Dark Waters leans a little harder on execution, though the gap is small.
Portal 2
Portal placement requires some precision but timing is lenient.
Risen 2: Dark Waters
Clunky melee with dirty-trick interrupts; modest demands
Risen 2: Dark Waters leans a little harder on reads, though the gap is small.
Portal 2
No opponent; puzzles are static.
Risen 2: Dark Waters
Scripted enemy behavior throughout
Risen 2: Dark Waters leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
Risen 2: Dark Waters
Light loot randomness only
Portal 2 leans clearly harder on optimization.
Portal 2
Solving the puzzle's logic IS the entire game.
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.