head to head
Neither is simply harder - they are hard in different places. Baba Is You demands more optimization; Fallout: New Vegas demands more execution.

Optimization · Pure Optimization
A rule-rewriting puzzle game that is pure deterministic problem solving.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.
Baba Is You leans Optimization (Pure Optimization); Fallout: New Vegas leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Baba Is You
No reflex or input precision involved.
Fallout: New Vegas
Shooter inputs with VATS assist
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Baba Is You
No opponent; puzzles are static.
Fallout: New Vegas
Scripted factions and enemies
Fallout: New Vegas leans clearly harder on variance.
Baba Is You
Puzzles fully deterministic with no hidden information.
Fallout: New Vegas
Some loot and crit RNG
Baba Is You leans clearly harder on optimization.
Baba Is You
Literally solvable — find the rule rewrite that wins.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
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.