head to head
Mortal Shell asks more than Red Dead Redemption 2, and the gap is execution.

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

Execution · Practiced Mastery
A cinematic open-world western leaning on shooting feel and atmosphere.
Mortal Shell leans Execution (Practiced Mastery); Red Dead Redemption 2 leans Execution (Practiced Mastery).
Mortal Shell leans clearly harder on execution.
Mortal Shell
Hardened parry timing demands inputs
Red Dead Redemption 2
Gunplay and Dead Eye reward aim and timing.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Shell
Scripted bosses
Red Dead Redemption 2
Enemies use scripted AI patterns, not adaptation.
Red Dead Redemption 2 leans a little harder on variance, though the gap is small.
Mortal Shell
Minor item randomness
Red Dead Redemption 2
Some hunting and weather randomness, lightly hidden.
Mortal Shell leans clearly harder on optimization.
Mortal Shell
Shell choice and upgrade routing matter
Red Dead Redemption 2
Loadouts and routes solvable but loose.
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.