head to head
Neither is simply harder - they are hard in different places. Riven: The Sequel to Myst demands more optimization; The Elder Scrolls V: Skyrim demands more execution.

Optimization · Pure Optimization
A cerebral puzzle-adventure that is essentially a logic puzzle to be fully solved.

Optimization · Pure Optimization
An open-world RPG mostly about discovering and stacking exploitable progression systems.
Riven: The Sequel to Myst leans Optimization (Pure Optimization); The Elder Scrolls V: Skyrim leans Optimization (Pure Optimization).
The Elder Scrolls V: Skyrim leans clearly harder on execution.
Riven: The Sequel to Myst
no reflex or physical skill required
The Elder Scrolls V: Skyrim
Combat is forgiving; aim and timing rarely punish you.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Riven: The Sequel to Myst
no opponent present in the game
The Elder Scrolls V: Skyrim
Enemy AI is scripted and exploitable; not adaptive.
The Elder Scrolls V: Skyrim leans clearly harder on variance.
Riven: The Sequel to Myst
fully deterministic puzzle world
The Elder Scrolls V: Skyrim
Loot tables and leveled lists inject mild randomness.
Riven: The Sequel to Myst leans clearly harder on optimization.
Riven: The Sequel to Myst
pure logic-puzzle solving, walkthrough-solvable
The Elder Scrolls V: Skyrim
Skill stacking, enchant loops, and exploits are deeply solvable.
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.