head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Optimization · Pure Optimization
An open world where build optimisation dissolves every combat challenge.

Optimization · Pure Optimization
A scripted RPG where pattern-learning and pacifist routing carry you.
The Elder Scrolls V: Skyrim - Special Edition leans Optimization (Pure Optimization); Undertale 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.
The Elder Scrolls V: Skyrim - Special Edition
Combat timing matters little; difficulty bends to your build
Undertale
Bullet-hell dodging in scripted patterns
Undertale leans a little harder on reads, though the gap is small.
The Elder Scrolls V: Skyrim - Special Edition
Draugr and dragons run scripted AI packages
Undertale
NPCs follow fixed dialogue trees, not adaptive
The Elder Scrolls V: Skyrim - Special Edition leans a little harder on variance, though the gap is small.
The Elder Scrolls V: Skyrim - Special Edition
Loot rolls and radiant quests add minor mapped randomness
Undertale
Minimal randomness in encounters
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
The Elder Scrolls V: Skyrim - Special Edition
Perk, crafting and stealth-archer loops trivially solve combat
Undertale
Learning fixed boss patterns and mercy routes wins
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.