head to head
Neither is simply harder - they are hard in different places. The Elder Scrolls V: Skyrim demands more execution; The Incredible Machine 3 demands more optimization.

Optimization · Pure Optimization
An open-world RPG mostly about discovering and stacking exploitable progression systems.

Optimization · Pure Optimization
A physics puzzle of engineering Rube Goldberg contraptions to solve fixed goals.
The Elder Scrolls V: Skyrim leans Optimization (Pure Optimization); The Incredible Machine 3 leans Optimization (Pure Optimization).
The Elder Scrolls V: Skyrim leans clearly harder on execution.
The Elder Scrolls V: Skyrim
Combat is forgiving; aim and timing rarely punish you.
The Incredible Machine 3
Placing parts is untimed, no dexterity needed
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
The Elder Scrolls V: Skyrim
Enemy AI is scripted and exploitable; not adaptive.
The Incredible Machine 3
Solo puzzle, nothing to predict
The Elder Scrolls V: Skyrim leans a little harder on variance, though the gap is small.
The Elder Scrolls V: Skyrim
Loot tables and leveled lists inject mild randomness.
The Incredible Machine 3
Physics is deterministic, no randomness
The Incredible Machine 3 leans clearly harder on optimization.
The Elder Scrolls V: Skyrim
Skill stacking, enchant loops, and exploits are deeply solvable.
The Incredible Machine 3
Engineering a working contraption from parts IS the game
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.