head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Plants vs. Zombies 2: It's About Time demands more variance.

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

Optimization · Pure Optimization
Lane-defense economy optimization against semi-randomised zombie waves.
Fallout: New Vegas leans Optimization (Pure Optimization); Plants vs. Zombies 2: It's About Time leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Plants vs. Zombies 2: It's About Time
Tapping plants quickly only matters in surge moments
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
Plants vs. Zombies 2: It's About Time
Zombie waves are announced patterns, not minds
Plants vs. Zombies 2: It's About Time leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Plants vs. Zombies 2: It's About Time
Wave compositions and plant-food drops add per-level randomness
Fallout: New Vegas leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Plants vs. Zombies 2: It's About Time
Lawn economy and plant placement are the solvable core
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.