Tobirama is so bad. Kishimoto basically took a dozen character traits and crammed them into this forgettable loser.
—Has a lesser form of Edo Tensei
—Has a lesser form of Flying Thunder God
—Invented the Shadow Clone Jutsu, which even fodder Jonin learn
—Low tier in Suiton
—Hated Uchiha but is...
It was a Hanzo 20+ years older that had "lost his edge", which was Mifune's (and ironically Hanzo's) point. Hanzo's ambition was no more so his will to fight was hampered. Put Hanzo in his prime and Mifune gets clowned.
Kabuto's flashback was phenomenal. It's the closest Kishi gets to world building the other nations and being forced to kill your own mom while she doesn't recognize you? Great stuff, all around.
Definitely not the right time for it. Dunno why Kishi did it mid fight when he already told us in...