When I say fantasy is easy, what I mean is that writing in a background you already know intimately is easier than figuring out every little thing about the background. It’s easiest to put this in terms of tech levels — if you’re going to write something at a medieval tech level, or a classical tech level, or an eighteenth century tech level, all you have to do is half a ton of research, most of which (if you’re me) you’ll already have done while thinking you were doing something else. The information is out there. All you have to do is integrate that information and the magic and the history into your background. There’s tons and tons of it, you’ve already absorbed lots of it unconsciously, so when you change anything it’s clear how it connects and what else changes with it.
Wherever you start — and I tend to start intuitively, with characters, who accrete their world and magic and technology (not to mention plot!) as they go — you can go forward from there. You know how long it takes to go 20 miles, at tech level, you know that 20 miles on flattish land is the radius a C.12 equivalent castle can defend (how far a horse can be ridden and still fight. “Flattish land” is why there are so many castles in Wales, which has no flat land to speak of…) you know how messages are sent and exactly how a castle wall is built. (Incidentally, if you don’t know how a castle wall is built, they’re supposed to be rebuilding some at Oystermouth this summer. I watched them the last time they were doing some there and it was very interesting.) You can change all those things, if you want to, but even so the implications of the tech you don’t stop and think about will all fit together naturally because they really did — unless you really screw up the way the magic integrates, or unless you didn’t do the research, it’ll all hang together because it’s all borrowed from the real and complex world.
When you write SF, you have to stop and figure out every single bit of that, and how it hangs together, and all the second order implications of the tech, and the problem with that, if you write intuitively starting the characters whose world is implicit, is that it makes it all incredibly slow, and for me, when I slow to a certain point, it stalls out.
Tolkien said what he liked was history “true or feigned”, and I think fantasy worlds are, more than anything, a chance to feign history.
When I write fantasy, (or, for that matter, alternate history, which is even more history) what I’m drawing from when I start making things up is history. When I write SF, to a large extent what I’m drawing on, to get the same 360 degree versimilitude, is other SF.
I don’t think there’s anything wrong with variations on a theme (Bach!!!) but I’m much more likely to do something original when I write fantasy, or alternate history, even though what I like to read best in all the world is someone landing for the first time on a planet that smells strange and has aliens.