head to head
Neither is simply harder - they are hard in different places. Cult of the Lamb: Cultist Edition demands more optimization; Grand Theft Auto V demands more reads.

Optimization · Practiced Mastery
Managing a cult's economy and rituals between light roguelike combat runs.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Cult of the Lamb: Cultist Edition leans Optimization (Practiced Mastery); Grand Theft Auto V 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.
Cult of the Lamb: Cultist Edition
Dodge-roll combat matters but stays forgiving
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Cult of the Lamb: Cultist Edition
Enemies and bosses are scripted, never adaptive
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Cult of the Lamb: Cultist Edition leans clearly harder on variance.
Cult of the Lamb: Cultist Edition
Weapon, curse and dungeon rolls vary each crusade
Grand Theft Auto V
Some mission and lobby randomness.
Cult of the Lamb: Cultist Edition leans clearly harder on optimization.
Cult of the Lamb: Cultist Edition
Cult economy, follower loops and doctrine planning dominate
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.