head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Immortals Fenyx Rising demands more optimization.

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

Optimization · Practiced Mastery
Breezy open-world adventure romping through Greek myth puzzles and monsters.
Grand Theft Auto V leans Execution (Practiced Mastery); Immortals Fenyx Rising leans Optimization (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Immortals Fenyx Rising
Dodge timing and aerial combos
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Immortals Fenyx Rising
Mythic beasts follow fixed movesets
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Immortals Fenyx Rising
Little randomness in the open world
Immortals Fenyx Rising leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Immortals Fenyx Rising
Vault puzzles, gear builds, and stamina routing
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.