Or you could just look at the facts: Minato's Assist mystery extend his original effects (interruption and tag) into and AoE + Paralysis + 30% dmg. Also he can dodge, and potentially use mystery every round.
Itachi, on the other hand, only gets the tag part transformed into an Aoe (not the buff removal) + no new effect + 30% dmg. Also he stays on a 3 round CD (that you cant even reset it with Lightning main's Killing Intent!), and he obviously cannot dodge (even worse, he's usually played in pos 3 or 4).

Seems fair to say his assist mystery is a bit underwhelming and could benefit from an upgrade. Not that it's ever gonna happen anyway.