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.

Execution · Practiced Mastery
A lean souls-like carried by hardened-parry timing and shell selection.

Execution · Practiced Mastery
A story-driven skateboarding game still won by trick execution.
Mortal Shell leans Execution (Practiced Mastery); Tony Hawk's Underground leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Shell
Hardened parry timing demands inputs
Tony Hawk's Underground
Combos plus off-board tricks keep the input ceiling high
Mortal Shell leans a little harder on reads, though the gap is small.
Mortal Shell
Scripted bosses
Tony Hawk's Underground
Story missions are scripted, not adaptive
Mortal Shell leans a little harder on variance, though the gap is small.
Mortal Shell
Minor item randomness
Tony Hawk's Underground
Fixed levels and mission scripts
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Shell
Shell choice and upgrade routing matter
Tony Hawk's Underground
Mission and score-line routing remain 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.