Why Nagato can't control Gedo Mazo very well

[Kurogane]

No, you get it wrong. Madara chose Nagato because he knew that Nagato could control the mazou very well. He had a senju lineage and the Rinnegan gave him an uchiha DNA. That's why he said "you can't control the mazou without senju and uchiha DNA"
Konan forbid him to use the mazou because she knew he would use the dragon soul jutsu which the side effect to the user's body. If Obito and Madara were about to use the same jutsu, i think the side effect would still happen

 

[Strict]

Madara's eyes count as Uchiha dna.

In this case Kakashi would have escaped from Tsukuyomi since having the Sharingan and Uchiha genes.

 

[VongolaX]

wow is that how you react to a joke? you must not be well liked at your school i take it

I knw it was a joke, thts how most of my friends and I communicate with each other, we insult each other and laugh about who came up with a
better insults.

 

[Great Toad Sage]

agreed ..no other proper explanation anyways