I like my SF very hard ("Incandescence" by Greg Egan is roughly where I'd say I'm comfortable, a plausible mechanism by which a pre-industrial civilisation might discover general relativity, that novel made me cry at the end) but even when he's being totally whimsical I really enjoyed Lem.
That aside,
I just have a regular understanding of most fields in science. I know a bit here and there. More astronomy.
My field is computer science, so I have some knowledge there.
What I wonder about is if the average scifi reader expects or even could tell if something is "science" and not fantasy. Huge absurd things of course.
I have read so many different descriptions on how FTL works. I dont think (ignorance on my part) that we have a solid theory for how it can be done.
- Warping of space/time (or higher dimension),
- "portals" left by "an ancient civilization"(that sort of evades the issue)
- wormholes
- "Taming a god"
- through special cracks in space/time that only a special navigator (species) can feel.
- improbability drive (I do love Douglas Adams).
and many more.
When you read can you take the existence of FTL on just being there, do you reject the ideas fully, or do you judge it on its merits if the description extrapolates current knowledge into a future where we can FTL? (if ever)
This is genre fiction, and genres have rules. For example, Romance is a genre of happily ever after endings. If you write what is ostensibly a Romance novel, but the heroine realises six pages from the end that her new lover is cheating on her and so she walks away unhappily and that's it, that's not a Romance novel, a Romance imprint would reject it, if you're a big name author they'd tell you to take it to a literary fiction house - otherwise go away and rewrite with a happy ending.
In SF the rule is "What if ...?", so you absolutely can have anything, unicorns, magic spells, faster than light travel, God can be real, but the story is about what else if that was so? That's where science comes into it. OK, so there are unicorns, what's special about them, just horses with a weird horn or anything more? Do people... ride unicorns? Eat them? Or maybe the unicorns eat people.
A fantasy kingdom with a rich gold mine can be dirt poor and yet money is somehow measured in gold, the thing they have plenty of, in fantasy you needn't explain, but in SF that's either a massive error or the core thesis of a novel.
On FTL specifically. I'm not a fan for reasons Charlie Stross explained when he gave up on writing a sequel I'd kinda wanted to read some day in a setting where he'd tried to tame FTL. FTL is time travel. So, you need to either embrace that, and have arbitrary time travel in your story (good luck producing a narrative you can write down) or come up with a water tight reason nobody ever does this. That's just a high cost it usually isn't worth it.
That said, the Clockwork Rocket series by Egan does just do time travel, but it also has different spatial dimensional layout, it's set somewhere way stranger than the setting for Incandescence, which is our universe albeit not somewhere humans could ever go. Still, I wasn't enormously happy with the outcome, time travel still ends up being sort of cheating even in the framework Egan creates. They do, as hoped, solve their impending disaster by going very, very fast though. Also they fix the patriarchy, which is way harder in a world where women inherently die during the equivalent of childbirth...