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 · Practiced Mastery
Landmark scripted FPS following Gordon Freeman through the Black Mesa disaster.

Optimization · Practiced Mastery
An open-world action game where hacking the city's infrastructure is your main weapon.
Half-Life leans Execution (Practiced Mastery); Watch Dogs leans Optimization (Practiced Mastery).
Half-Life leans a little harder on execution, though the gap is small.
Half-Life
Combat aim and infamous platforming sections
Watch Dogs
Cover shooting and driving at modest difficulty
Watch Dogs leans a little harder on reads, though the gap is small.
Half-Life
Marines flank by script, not by modelling you
Watch Dogs
Online invasions add a small human element
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life
Set pieces surprise only on first run
Watch Dogs
Guard placements are knowable on approach
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life
Resource routing and encounter knowledge for speedruns
Watch Dogs
Hacking chains and camera routes make stealth 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.