head to head
Neither is simply harder - they are hard in different places. Need for Speed: Rivals demands more variance; Rocket League demands more reads.

Execution · Pure Mechanics
Open-world racer where banking risky speedpoints against human cops drives the tension.

Execution · Pure Mechanics
Mechanical car-soccer where execution is the ceiling.
Need for Speed: Rivals leans Execution (Pure Mechanics); Rocket League leans Execution (Pure Mechanics).
Rocket League leans clearly harder on execution.
Need for Speed: Rivals
Open-road control with pursuit tech timing
Rocket League
Aerials, flicks, dribbles, pure mechanical ceiling
Rocket League leans clearly harder on reads.
Need for Speed: Rivals
AllDrive mixes human cops and racers in
Rocket League
Rotations and 50-50 reads against humans
Need for Speed: Rivals leans clearly harder on variance.
Need for Speed: Rivals
Dynamic open-world encounters and weather
Rocket League
Almost deterministic physics
Need for Speed: Rivals leans a little harder on optimization, though the gap is small.
Need for Speed: Rivals
Risk-versus-bank speedpoint economy
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.