head to head
FlatOut: Ultimate Carnage asks more than Runbow, and the gap is optimization.

Execution · Pure Mechanics
Keeping speed through destruction-racing chaos that punishes clean-lap plans.

Execution · Pure Mechanics
Reaction-speed platform racing where shifting colours keep redrawing the level under you.
FlatOut: Ultimate Carnage leans Execution (Pure Mechanics); Runbow leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
FlatOut: Ultimate Carnage
Holding racing lines through wrecks and ragdoll chaos tests hands
Runbow
Fast platform racing through vanishing platforms is pure reaction
Runbow leans a little harder on reads, though the gap is small.
FlatOut: Ultimate Carnage
Rival drivers ram predictably, light multiplayer jockeying
Runbow
Light jostling with human racers, but little deep prediction
Runbow leans a little harder on variance, though the gap is small.
FlatOut: Ultimate Carnage
Debris and pileups scramble any planned lap
Runbow
Colour swaps and party chaos randomise each race
FlatOut: Ultimate Carnage leans clearly harder on optimization.
FlatOut: Ultimate Carnage
Car upgrades and track shortcuts are mappable
Runbow
Minimal systems - momentum tricks and level memory only
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.