Shinryu You're getting confused between "Senju", which is Hashirama/Tobirama/Tsunade's family, and "Senjutsu" which means Sage Chakra (Hashirama can use both).
Dead Madara, having gained Hashirama's cells, used Mokuton (wood element) jutsu, but he never used Sage techniques. The ability to use Mokuton jutsu is hereditary, so having Hashirama's cells and also knowing the hand seals for all of Hashirama's jutsu (because of the Sharingan), it's not surprising that Madara was able to unlock the Senju's Mokuton right away.
Sage techniques are different-- and the ability to become a sage and absorb natural energy is one that's learned through training; something that couldn't be copied just by seeing it with the Sharingan.
It was only in this most recent battle was it officially disclosed that Hashirama is a sage-- though how he has sage powers, no one knows, since he's the first character to not have gained the ability through training with Frogs/Snakes.
Anyway, through Naruto's training, Naruto's battle with pain (and one of the pains turning into a frog), and just seeing all the fucked up shit that happens to people trying to use Jugo's curse seal, we know Natural energy can be incredibly unstable and cause all sorts of shit.
That's why Madara was like, "Oh, so this is the fabled 'sage chakra'. Controlling this is supposed to be difficult? Eh, whatever."