As an aside, I haven't read a lot of it.
But you can tell just by the implementation of it that something was off. If they meant something theologically sound, it was overshadowed by the aftermath — 50 years of crumbling parishes, sinking vocations, and bad, neo-Marxist theology.
I personally don't know if the NO is invalid or not; it's not something I feel personally qualified to give an answer on.
But it's a stretch to say that anything good came out of Vatican II.