I’ve read lots of science fiction and lots of fantasy over the years. Most times the line between the two is pretty clear. Magic and dragons? Fantasy. Rocket ships and ray guns? Science fiction.
However, some writers blur the genres. The best writers can create hybrid genres of their own. Take Anne McCaffery’s dragonriders books. At first they read like non-magical fantasy. As the stories wind on, the reader finds a good bit more science fiction, but with the fantastical element of humans able to connect with dragons telepathically.
The movie Interstellar starts out as good science fiction, but soon fantastical interpretations of scientific principles crop up which takes it into some extremely speculative areas which are not backed by scientific theory. This is the main reason I did not enjoy what was otherwise a pretty good movie.
What about traveling faster than light? We have suspicions that theory may allow this in certain extreme cases, but so far it is only speculation. So no warp drives in science fiction? No, I think faster than light travel is a well accepted idea in the genre and is perfectly okay. Too much fanciful science or technology can be a problem in science fiction, but sometimes you need to move the story. If judicious use of something like a faster than light drive does the trick, well and good.
Another serious flaw occurs when a science fiction author dwells on their fanciful technology trying to make it more "believable." In the science fiction of the first half of the twentieth century authors sometimes took great pains to make their idea of advanced technology seem realistic. H. G. Wells did this in books like From the Earth to the Moon. Skip this. You’ve got an FTL drive? Great. Use it to go somewhere, but skip the tour of the engine room.
Caution: Unless it’s necessary to the plot, do not spend more than two sentences telling the reader how something you made up works. It just does. OK?
One additional note, and this time it’s on the right way to do it. The science fiction book and movie “The Martian" is giving scientific fiction a good name. Andy Weir’s main character approaches problems with the attitude of a scientist. He works hard to figure it out a solution. Of course the Watney character knows the facts about how it all works, but he has to figure out how to succeed with what he has. This was the reason why the Apollo 13 mission (and the movie) was such a great story. Here's what you've got. Here's what you have to do. Now, how do you do it?