head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more optimization; Risen 3: Titan Lords demands more variance.

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

Optimization · Pure Optimization
Build planning and faction routing through a fixed open world with modest combat demands.
Portal 2 leans Optimization (Pure Optimization); Risen 3: Titan Lords leans Optimization (Pure Optimization).
Risen 3: Titan Lords leans a little harder on execution, though the gap is small.
Portal 2
Portal placement requires some precision but timing is lenient.
Risen 3: Titan Lords
Dodge-and-strike combat punishes carelessness moderately
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal 2
No opponent; puzzles are static.
Risen 3: Titan Lords
Monsters and humans run fixed AI routines
Risen 3: Titan Lords leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
Risen 3: Titan Lords
World state is scripted; loot variance is minor
Portal 2 leans clearly harder on optimization.
Portal 2
Solving the puzzle's logic IS the entire game.
Risen 3: Titan Lords
Faction choice, build planning and quest routing dominate 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.