head to head
Neither is simply harder - they are hard in different places. Forza Horizon 3 demands more optimization; Rocket League demands more reads.

Execution · Practiced Mastery
An open-world arcade racer across Australia where car control and tuning carry every event.

Execution · Pure Mechanics
Mechanical car-soccer where execution is the ceiling.
Forza Horizon 3 leans Execution (Practiced Mastery); Rocket League leans Execution (Pure Mechanics).
Rocket League leans clearly harder on execution.
Forza Horizon 3
Car control and racing lines at speed
Rocket League
Aerials, flicks, dribbles, pure mechanical ceiling
Rocket League leans clearly harder on reads.
Forza Horizon 3
Drivatars mimic but don't truly adapt
Rocket League
Rotations and 50-50 reads against humans
Forza Horizon 3 leans a little harder on variance, though the gap is small.
Forza Horizon 3
Minor traffic and event variation
Rocket League
Almost deterministic physics
Forza Horizon 3 leans clearly harder on optimization.
Forza Horizon 3
Tuning, car choice, and route knowledge
Rocket League
Training routines, rotation theory
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.