head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Into the Breach demands more optimization.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.

Optimization · Pure Optimization
A perfect-information tactics puzzle solved one turn at a time.
Fallout: New Vegas leans Optimization (Pure Optimization); Into the Breach leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Into the Breach
No mechanical inputs
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
Into the Breach
Enemy moves telegraphed fully each turn
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout: New Vegas
Some loot and crit RNG
Into the Breach
Light spawn and squad randomness
Into the Breach leans clearly harder on optimization.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Into the Breach
Pure puzzle - perfect-information chess each turn
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.