>I think that the assumption that those 20th century projects failed because they were utopian is unsourced

Yes, but this is not about their eventual failure, but about their Dystopianism, which was part of their programme. Lenin explicitly stated that his "Jacobian party would never reject terror, nor could it do so". Lenin espoused state terror and mass murder, justified by "the greater good". If that is not idealism causing Dystopia right here, I don't know what is.

Cf. https://en.wikipedia.org/wiki/Red_Terror