What a sentence owes you
Not all hard sentences are hard for the same reason, and the difference is a moral one rather than a matter of taste.
There is a kind of sentence that stops you, and you go back, and on the third pass it opens — and you find that it needed every one of its clauses. And there is a kind of sentence that stops you, and you go back, and on the third pass you find that it is simply badly built.
Both are difficult. They are not the same thing.
Difficulty that belongs to the subject
Some things are genuinely hard to say. A thought with four conditions on it cannot be delivered in a sentence with one clause; compressing it would not simplify the idea, it would misrepresent it.
When a sentence is difficult because its content is, the reader’s effort is rewarded — not with pleasure necessarily, but with the actual thought, intact. That transaction is fair. The reader paid attention and received something that could not have been bought more cheaply.
Difficulty that belongs to the writer
The other kind is a sentence that is hard because its author had not finished thinking.
The clauses are in the order they occurred rather than the order they are needed. The subject arrives late for no reason. A word is doing two jobs and the reader has to hold both until the paragraph resolves, and then it does not resolve.
This is also difficult. But the effort buys nothing, because the compression was not in the idea — it was in the writer’s unwillingness to spend another hour. The cost was moved rather than removed, and it was moved onto someone who did not agree to pay it and, crucially, cannot tell from the inside which kind of sentence they are in.
That last part is what makes it a question of obligation rather than of style. The reader cannot audit you. They will assume the difficulty is theirs.
The unfair advantage of difficulty
Hard prose is also flattering, which is the uncomfortable part.
A reader who works through a difficult passage and arrives somewhere tends to credit the passage. The effort itself produces a feeling of significance, and that feeling attaches to whatever caused the effort — whether or not anything was actually there. Every writer who has ever been vague has benefited from this at some point, and most of us know it.
The test I use
Not “is this hard to read”, which proves nothing, but: can I say what the difficulty is buying?
If the answer is “it is holding four conditions in play, and dropping any of them would be a lie”, the sentence stays. If the answer is “it sounds more serious this way”, it goes — and it goes even when, especially when, I like it.
I fail this test regularly. But having it means the failures are decisions rather than accidents, and I would rather have written a plain sentence I chose than a difficult one I did not.