> Not explaining something is not the same as ignoring it.

No, that's pretty much the definition of it.

> If a society has advanced medical technology where changing your body is not just possible but broadly available, then it follows that they have solved any issues with rejection and adaptation.

No, that is just explaining away poor writing. Explaining necessary details makes the difference between good or bad storytelling.

> Scifi is about 'what if?' and how that affects people.

Starting with ignoring the first obvious consequences is not exploring how something affects people, it's just wishful thinking.

> Similarly, we don't need to know how the huge space station capable of destroying a whole planet in a single shot works (unless you are a rebel princess), just that it does.

If Star Wars would be SciFi, then we should get some good enough explanation for this. People are disputing about those details to great lengths for good reasons.

> Explaining necessary details makes the difference between good or bad storytelling.

Only when the details you are explaining are relevant to the story you want to tell and the themes you want to cover.

In The Left Hand of Darkness, Le Guin explores a planet populated by an offshoot of humans who have developed a genderless existence where they experience sexual characteristics only once a month and are genderless the rest of the time.

The book does not explain how this works biologically or why this came about evolutionarily, because that is not the point. The interest of the author was to explore the cultural and sociological implications of this situation. If a group of humans lived without gender most of the time, how would this affect their culture and society? And what does that in turn say about our own gendered society?

Diving into the biological nitty-gritty of this fictional scenario would distract from the social themes the author was trying to explore.

There were probably a few more sentences hand waving these sorts of details in the books, by the time they got mentioned you were probably more interested in worrying about the Moon-wide epidemic of suicide that the Moon’s governing AI had tasked the book’s protagonist with discovering the cause with, after the protagonist recovered from being brought back in a fresh clone after succumbing to it.

That’s the plot of Steel Beach, if you want to go see what happens next and how much time Varley actually spent on the details of this stuff.