head to head
Neither is simply harder - they are hard in different places. Fallout 76 demands more variance; Portal 2 demands more optimization.

Optimization · Pure Optimization
A live-service Fallout where perk-card builds and CAMP crafting drive progression.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Fallout 76 leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Fallout 76 leans a little harder on execution, though the gap is small.
Fallout 76
Shooter inputs in live combat
Portal 2
Portal placement requires some precision but timing is lenient.
Fallout 76 leans clearly harder on reads.
Fallout 76
PvP exists but most play is PvE
Portal 2
No opponent; puzzles are static.
Fallout 76 leans clearly harder on variance.
Fallout 76
Legendary and event loot rolls
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans clearly harder on optimization.
Fallout 76
Perk cards, CAMP, build crafting key
Portal 2
Solving the puzzle's logic IS the entire game.
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.