head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Risk of Rain Returns demands more variance.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Optimization · Practiced Mastery
Remake of the original Risk of Rain with new survivors and tighter action.
Grand Theft Auto V leans Execution (Practiced Mastery); Risk of Rain Returns leans Optimization (Practiced Mastery).
Risk of Rain Returns leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Risk of Rain Returns
Platforming and aiming demand sharper inputs than original.
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Risk of Rain Returns
Scripted enemy behaviour throughout.
Risk of Rain Returns leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
Risk of Rain Returns
Random item pool defines builds run to run.
Risk of Rain Returns leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Risk of Rain Returns
Survivor kits and routing reward optimization.
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.