The original versions of the stories did have more of a fairy-tale feel, didn't they? Like Beren being captive in the castle with the giant cat? But you've also got things, like the Fall of Gondolin, that go perfectly with the tone of Silmarillion...
I would agree with that. As I was reading "The Fall of Gondolin" in the Lost Tales I kept thinking that I wished JRRT had been able to fully re-write the story for The Silmarillion. The story fragment of "Tuor and the Fall of Gondolin" as presented in Unfinished Tales makes me think it would have been something incredible to read in full, on a par with anything in LOTR.