-Hagoromo created the tailed beast instead of just destroying the Juubi's chakra like Sasuke was going to do. He knew that the Juubi's revival was possible, yet he didn't destroy that possibility when he had the chance.
-Hagoromo didn't bother to tell Naruto and Sasuke how to dispel MT before confronting Madara. It was only by sheer luck that he returned to the world of the living to tell them.
-Hagoromo gave Naruto and Sasuke a seal that would create a moon. Had Naruto and Sasuke seal Madara in chapter 674(which they almost did) a large chunk of the planet would have been destroyed. They could have just defeated Madara the same way they did with Obito.
-Hagoromo thinks that Naruto and Sasuke should duke it out when the whole world is becoming Zetsu. Both of them almost die, putting the world in endanger.
-Hagoromo didn't bother to tell Naruto and tailed beast about Sasuke's plan. Endangering them.
-Hagoromo ruined the next generation surpassing the previous one theme.
-Hagoromo is a walking retcon that makes the entire story idiotic.
-Instead of desroying the juubi, he separated its power and guided the bijuu as if they were pets. The Juubi's power is limitless, it can't just "be destroyed". Even if you kill the bijuu they simply respawn after a period of time. They, as well as the juubi, can only be sealed.
-He gave them immeasurable power, they used it to defeat Madara and Kaguya...
What more was necessary? everything was taken care off..
- So a large chunk of the Earth would be taken. Point? It's a lot better than the whole Earth. Not to mention the entire Shinobi alliance was required when dealing with Obito and Zetsu and Kaguya were still roaming.
It worked out for the better.
-If they didn't "duke it out" the cycle of hatred would have continued and the world would be right back in danger. Also the people caught in tsuki no mi don't turn into zetsu until they're chakra is drained.. No one is around to drain they're chakra. Also Hagoromo had no control over the two fighting each other. He couldn't stop them or make them. Maybe by taking the chakra back, but they still would have fought.
- Would telling Naruto and co. about his plan stop Sasuke at all? No. What would them knowing prevent?
Hagoromo is an existence outside the realm of their world, in a sense. He can't tamper with the natural order of things just because he happens to be there.
-How did he ruin any theme? The idea of the generation surpassing the previous is still prevalent. Naruto has surpassed all of his masters and his master's masters.
You're obviously just reaching for bad things to point out so that you can gain some kind of approval from others it seems..