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 · Practiced Mastery
Open-world Stone Age action game about hunting and conquering tribal outposts with tamed beasts.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Far Cry Primal leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Far Cry Primal leans a little harder on execution, though the gap is small.
Far Cry Primal
Bow aim, melee timing, beast control under pressure
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans a little harder on reads, though the gap is small.
Far Cry Primal
Scripted tribal AI never models the player
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Far Cry Primal leans a little harder on variance, though the gap is small.
Far Cry Primal
Wildlife encounters and patrol spawns are random
Grand Theft Auto V
Some mission and lobby randomness.
Far Cry Primal leans a little harder on optimization, though the gap is small.
Far Cry Primal
Outpost routes, crafting economy, beast taming order
Grand Theft Auto V
Heists, money loops, builds learnable.
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.