head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Competitive PvP
Supercell's snappy top-down team brawler built around three-minute objective fights.

Execution · Competitive PvP
Stick execution and reading human opponents within a meta-driven system.
Brawl Stars leans Execution (Competitive PvP); FIFA 18 leans Execution (Competitive PvP).
FIFA 18 leans a little harder on execution, though the gap is small.
Brawl Stars
Aim, dodging, and ability timing in real time
FIFA 18
Perfect stick skill and timing dominate dribbles and finishing
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Brawl Stars
Human teams rotate, bait supers, and adapt
FIFA 18
Human opponents bait, switch tactics and counter your habits
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Brawl Stars
Bush ambushes and power-cube spawns hide information
FIFA 18
Bounce physics and pack luck add measured chaos
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Brawl Stars
Brawler matchups and map strategy are learnable
FIFA 18
Formation, squad chemistry and meta mechanics reward study
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.