head to head
Neither is simply harder - they are hard in different places. Fortnite demands more variance; Warcraft III: Reign of Chaos demands more optimization.

Execution · Grand Arena
Build-fight battle royale with deep mechanics.

Optimization · Grand Arena
Blizzard's hero-centric RTS where build-order theory meets surgical unit micro against humans.
Fortnite leans Execution (Grand Arena); Warcraft III: Reign of Chaos leans Optimization (Grand Arena).
Fortnite leans a little harder on execution, though the gap is small.
Fortnite
Build-edit-aim combos punish slow hands
Warcraft III: Reign of Chaos
Micro of heroes and units decides fights
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Fortnite
Box fights are pure adaptive mind games
Warcraft III: Reign of Chaos
Scouting and out-guessing a human opponent's tech
Fortnite leans clearly harder on variance.
Fortnite
Loot drops, storm, item rotations vary
Warcraft III: Reign of Chaos
Creep routes and item drops add randomness
Warcraft III: Reign of Chaos leans clearly harder on optimization.
Fortnite
Loadouts, mats economy, rotations solvable
Warcraft III: Reign of Chaos
Build orders, creeping routes, and matchup theory run deep
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.