head to head
Neither is simply harder - they are hard in different places. Overwatch demands more execution; Supreme Commander 2 demands more optimization.

Execution · Grand Arena
Team hero shooter blending FPS mechanics with ability economy mind games.

Optimization · Grand Arena
Large-scale RTS testing economic optimization, multitask speed and prediction of human build choices.
Overwatch leans Execution (Grand Arena); Supreme Commander 2 leans Optimization (Grand Arena).
Overwatch leans clearly harder on execution.
Overwatch
Hero aim, movement, and ability timing
Supreme Commander 2
Macro APM across huge armies separates skirmish winners
Overwatch leans a little harder on reads, though the gap is small.
Overwatch
Ult tracking, flanks, and baiting human teams
Supreme Commander 2
Multiplayer rewards scouting and predicting a human's tech switches
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Overwatch
Minor spread and matchmaking noise
Supreme Commander 2
Open maps leave little hidden beyond fog of war
Supreme Commander 2 leans clearly harder on optimization.
Overwatch
Team comps, ult economy, and positioning theory
Supreme Commander 2
Economy ratios, build orders and research routing are deeply solvable
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.