How difficult is it really to get a sci-fi/fantasy/horror novel published?

The number which is normally quoted is that about 1 in 1000 finished polished novels gets bought by a commercial publisher. Now many of those will be complete dross...but even so, it's not good odds. I know some really _very_ good writers who've tried and failed to get their fantasy novels published.

So yes, it is difficult.

I'm assuming you mean commercially, of course. If you want to vanity publish, it's trivially easy these days to do so online using a site like lulu.

Childhoods end Arther C Clark commencing place series via Isac Asmove, The Hobit greater fantasy The mote in god eye Gordon R Dickson, The Dune series Frank Hurbert,1984 George Orwell.

Quit difficult. The competition in that genre is very tough. It helps to go to science fiction conventions and mingle with the editors and publishers.