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

Optimization · Pure Optimization
Villainous dungeon management game about luring monsters and crushing do-gooder heroes.

Optimization · Pure Optimization
A scripted RPG where pattern-learning and pacifist routing carry you.
Dungeon Keeper 2 leans Optimization (Pure Optimization); Undertale leans Optimization (Pure Optimization).
Undertale leans a little harder on execution, though the gap is small.
Dungeon Keeper 2
Slap-and-drop micro and possession combat need quick hands.
Undertale
Bullet-hell dodging in scripted patterns
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Dungeon Keeper 2
Hero waves and rival keepers run on scripts.
Undertale
NPCs follow fixed dialogue trees, not adaptive
Dungeon Keeper 2 leans a little harder on variance, though the gap is small.
Dungeon Keeper 2
Hero party timing and creature moods vary mildly.
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.
Dungeon Keeper 2
Dungeon layout, creature economy and research are solvable.
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.