head to head
Neither is simply harder - they are hard in different places. Bunch of Heroes demands more execution; Grand Theft Auto: San Andreas demands more optimization.

Execution · Practiced Mastery
A top-down co-op shooter testing aim and survival against alien swarms.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Bunch of Heroes leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Bunch of Heroes leans clearly harder on execution.
Bunch of Heroes
Twin-stick aiming and dodging swarms is the core
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Bunch of Heroes leans a little harder on reads, though the gap is small.
Bunch of Heroes
Enemies swarm on scripted AI, no adaptation
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Bunch of Heroes leans a little harder on variance, though the gap is small.
Bunch of Heroes
Enemy spawns and waves add some randomness
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Bunch of Heroes
Upgrade and co-op positioning choices matter lightly
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
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.