Very few original movies will end up being major successes. You look at the top grossing movies of every year and they are mostly franchises, adaptations, and remakes. This has been true for decades and applies to all genres.

Yes, big movies are in general a risk. Sci-fi movies often have high budgets due to nature of the genre so they come with risk, but I don't think the genre fails at an abnormally large rate compared to other genres.