And can blow up in your face if you aren't careful! A big part of the cost, of course, is any reasonable semblance of credulity. I still really like The Martian!
Decades ago I wrote a short story about an interstellar expedition in which the voyagers return to earth after having aged considerably less than earth's inhabitants. I should find that, maybe I can write a decent screenplay based on it. I was aiming for credulity. Of course, I don't have a design for a propulsion system intense enough to take advantage of relativistic time differential. That's just an assumed given in my story. The Martian has no such flaw. I was in awe of that book and am totally incapable of creating anything that compares to it. Well, I'm not a space travel freak like Andy Weir.
Star Wars vs Star Trek.
In Star Wars, "engines" and "hyperdrive" made people go from one place to another. They weren't really used in the story, except when the hyperdrive needed to break.
In Star Trek, they spend more time getting into the hardware behind the engines, and the weird things they can do.
So your interstellar travelers could have also used "engines" to make them go fast.
I guess if they're not really mentioned at all, it's not as big of a problem. You assume the subsystem is present and it works well; the rest of the story is the focus, not the technology.
But if a writer mentions the engines a lot and throws a lot of what is essentially magic into it, or else random terms that only
sound like they belong there, that's where things fall apart. ("If they did go into light Even traversing a tiny galaxy at light speed would take many
many years. Saying "hyperspeed" or "hyperdrive" would have alleviated the major gripe from that line. I guess if you're Lucas or a Hollywood writer, light is "really fast" and space is "a little on the big side," and math is hard)