> Good subtitles are more akin to interpretation than translation. You have less space to fit, timing is important, you need to translate idioms, etc.
Timing particularly! You'll often get a mystery where the detective says "And the killer is … [long, dramatic pause to survey the room] … the butler!", but the subtitles, displayed at the beginning of the sentence, just say "And the killer is the butler!"
I've noticed HBO seems to have the highest quality subtitles in terms of timing and position on screen relative to the character speaking.
Then there's other shows I watch on Netflix will do that exact thing you mentioned - spoiling the punchline because the subtitle appears before a single word is spoken.
Oh, I forgot that Hulu has an infuriating thing where, if you turn on English-language subtitles for a show that's mostly in English (as those of us with poor hearing often do), and the show has burnt-in subtitles for portions of speech in some other language, then the burnt-in subtitles will be overwritten by a subtitle reading, say, "[Speaking in French]."
Timing particularly! You'll often get a mystery where the detective says "And the killer is … [long, dramatic pause to survey the room] … the butler!", but the subtitles, displayed at the beginning of the sentence, just say "And the killer is the butler!"