head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Practiced Mastery
Aim and movement through a fully scripted campaign that route knowledge solves.

Execution · Practiced Mastery
Magic-as-guns FPS campaign through a war between battlemage legions.
Half-Life: Source leans Execution (Practiced Mastery); Immortals of Aveum leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life: Source
Aim, movement and platforming through Black Mesa's gauntlets
Immortals of Aveum
Spell-slinging aim and dodge timing
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life: Source
Marines and aliens run impressive but fixed scripts
Immortals of Aveum
Enemy mages run scripted AI
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life: Source
Fully authored campaign, nothing random
Immortals of Aveum
Minor loot randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life: Source
Route knowledge and resource management trivialise replays
Immortals of Aveum
Sigil loadouts and talent-tree choices
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.