I assume the basic issue is that by the time they've decided how big the text is (and therefore if/where they need hyphenation), they've already decided to use

I assume the basic issue is that by the time they've decided how big the text is (and therefore if/where they need hyphenation), they've already decided to use the ligature. Dynamically deciding whether or not to use the ligature when its at the linebreak seems expensive... refusing to do a ligature in the presence of a soft hyphen and otherwise treating the ligature as a single unit that cannot be broken seems like the more tractable way out of the problem... but I'm sure I'm avoiding or unaware of some additional piece of complexity.