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

Execution · Practiced Mastery
Loot-driven sequel sending Death through sprawling dungeons to clear War's name.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Darksiders II leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Darksiders II leans a little harder on execution, though the gap is small.
Darksiders II
faster combo combat as Death
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Darksiders II
no adaptive opponent
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Darksiders II
loot drops add light randomness
Grand Theft Auto V
Some mission and lobby randomness.
Darksiders II leans clearly harder on optimization.
Darksiders II
skill trees and dungeon puzzles reward planning
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.