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.

Execution · Pure Mechanics
A run-and-gun boss rush about dodging scripted patterns with tight inputs.

Execution · Practiced Mastery
Punishing platforming execution over learnable boss orders and part builds.
Cuphead leans Execution (Pure Mechanics); Mega Man X6 leans Execution (Practiced Mastery).
Cuphead leans a little harder on execution, though the gap is small.
Cuphead
Tight dodging and parry timing decide every boss.
Mega Man X6
Cruel platforming and boss windows demand frame-tight inputs
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Cuphead
Bosses cycle scripted phases; no adaptation to player.
Mega Man X6
Bosses are fixed pattern sets
Mega Man X6 leans a little harder on variance, though the gap is small.
Cuphead
Minor attack-order shuffling between phases.
Mega Man X6
The Nightmare System shuffles level hazards between runs
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Cuphead
Loadouts and boss phase order are fully learnable.
Mega Man X6
Weapon orders, armor parts and route knowledge are essential
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.