Is it easier to create a fantastical plot than a realistic one?

No, it’s a lot harder.

See, with a realistic plot, you know what the limitations are. And your readers know them, too.

With speculative fiction, you need to carefully construct the limitations yourself, and then communicate them to the reader without being too obvious about it. After all, your characters live in this world and take it for granted, so you can’t just point stuff out.

About the most common mistake – and even large franchises like Star Wars and Star Trek fall into this trap occasionally – is to have a neat gadget (or spell or magical item, in the case of fantasy) which, once you think it through, could short-circuit the entire plot. This is why communicators and transporters fail or suffer “interference” so often in Star Trek, for instance: the characters could easily escape any predicament by contacting the Enterprise and asked to be beamed up, otherwise!

It’s a lot easier to create plot holes for yourself in speculative fiction.