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
Build planning and faction routing through a fixed open world with modest combat demands.

Optimization · Pure Optimization
An open world where build optimisation dissolves every combat challenge.
Risen 3: Titan Lords leans Optimization (Pure Optimization); The Elder Scrolls V: Skyrim - Special Edition 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.
Risen 3: Titan Lords
Dodge-and-strike combat punishes carelessness moderately
The Elder Scrolls V: Skyrim - Special Edition
Combat timing matters little; difficulty bends to your build
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Risen 3: Titan Lords
Monsters and humans run fixed AI routines
The Elder Scrolls V: Skyrim - Special Edition
Draugr and dragons run scripted AI packages
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Risen 3: Titan Lords
World state is scripted; loot variance is minor
The Elder Scrolls V: Skyrim - Special Edition
Loot rolls and radiant quests add minor mapped randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Risen 3: Titan Lords
Faction choice, build planning and quest routing dominate power
The Elder Scrolls V: Skyrim - Special Edition
Perk, crafting and stealth-archer loops trivially solve combat
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.