head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Marvel Ultimate Alliance 3: The Black Order demands more optimization.

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

Optimization · Practiced Mastery
A Switch team brawler where alliance bonuses and stagger-bar math outweigh execution.
Grand Theft Auto V leans Execution (Practiced Mastery); Marvel Ultimate Alliance 3: The Black Order 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.
Marvel Ultimate Alliance 3: The Black Order
Stagger combos and dodges need modest hands.
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Marvel Ultimate Alliance 3: The Black Order
Bosses run fixed stagger scripts.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Some mission and lobby randomness.
Marvel Ultimate Alliance 3: The Black Order
ISO-8 drops add light randomness.
Marvel Ultimate Alliance 3: The Black Order leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Marvel Ultimate Alliance 3: The Black Order
Team bonuses, ISO-8 loadouts and stagger math dominate.
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.