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.

Optimization · Pure Optimization
A 2D sandbox of procedurally generated planets driven by crafting progression.

Optimization · Practiced Mastery
Routing efficient digs and upgrades through a randomised mine.
Starbound leans Optimization (Pure Optimization); SteamWorld Dig leans Optimization (Practiced Mastery).
SteamWorld Dig leans a little harder on execution, though the gap is small.
Starbound
Light platforming and combat, generous margins
SteamWorld Dig
Digging and platforming need modest precision
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Starbound
Procedural NPCs follow simple fixed behaviours
SteamWorld Dig
Enemies follow simple fixed patterns
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Starbound
Procedural planets and loot keep early exploration unknown
SteamWorld Dig
Randomised mine layouts hide ore and hazards each run
Starbound leans a little harder on optimization, though the gap is small.
Starbound
Gear tiers, farming, and progression routes dominate
SteamWorld Dig
Upgrade routing and efficient dig paths 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.