head to head
Neither is simply harder - they are hard in different places. Divekick demands more reads; Naruto Shippuden: Ultimate Ninja Storm 4 demands more execution.

Reads · Pure Mind Games
A two-button parody fighter that strips fighting games down to pure one-hit-kill spacing reads.

Reads · Competitive PvP
Anime arena fighter where managing the substitution gauge is the real duel.
Divekick leans Reads (Pure Mind Games); Naruto Shippuden: Ultimate Ninja Storm 4 leans Reads (Competitive PvP).
Naruto Shippuden: Ultimate Ninja Storm 4 leans clearly harder on execution.
Divekick
Two buttons, one-hit kills; almost no physical ceiling.
Naruto Shippuden: Ultimate Ninja Storm 4
Simple inputs; movement and substitution timing matter
Divekick leans clearly harder on reads.
Divekick
Pure spacing yomi against an adapting human is the entire game.
Naruto Shippuden: Ultimate Ninja Storm 4
Versus humans, baiting substitutions is the core game
Naruto Shippuden: Ultimate Ninja Storm 4 leans a little harder on variance, though the gap is small.
Divekick
Fully deterministic.
Naruto Shippuden: Ultimate Ninja Storm 4
Minor support-assist chaos
Naruto Shippuden: Ultimate Ninja Storm 4 leans a little harder on optimization, though the gap is small.
Divekick
Character angles and gem choices are quickly mapped.
Naruto Shippuden: Ultimate Ninja Storm 4
Team composition and matchup knowledge
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.