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

Optimization · Practiced Mastery
Ezio rebuilds the Assassin order in Rome, with a cult-classic mind-game multiplayer attached.

Optimization · Practiced Mastery
An open-world pirate adventure where upgrading the Jackdaw matters more than the blade.
Assassin's Creed Brotherhood leans Optimization (Practiced Mastery); Assassin's Creed IV: Black Flag leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Assassin's Creed Brotherhood
Kill-streak combat is stylish but easy
Assassin's Creed IV: Black Flag
Naval broadsides and boarding need modest timing
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Assassin's Creed Brotherhood
Hide-and-hunt multiplayer ran on human mind-games
Assassin's Creed IV: Black Flag
Multiplayer deception mode; campaign is scripted
Assassin's Creed IV: Black Flag leans a little harder on variance, though the gap is small.
Assassin's Creed Brotherhood
Fixed campaign content
Assassin's Creed IV: Black Flag
Weather, convoys, and hunting spawns
Assassin's Creed IV: Black Flag leans a little harder on optimization, though the gap is small.
Assassin's Creed Brotherhood
Recruit management and shop renovation economy
Assassin's Creed IV: Black Flag
Ship upgrades and fleet economy drive progress
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.