I would venture to say that the reason it’s not obvious whether “extremely well-received memo” needs a hyphen or not is because it doesn’t really make any difference to comprehension. The mental interpreter that quickly and unconsciously pulls meaning out of word groupings is not going to be led astray by the absence of a hyphen, because a “well-received memo” is substantially the same thing as a memo that has been received well, as distinctly unlike “the best-known author” and the potentially different meaning of “the best known author”. My own inclination would be to hyphenate on the basis that the key concept here is praise for the memo, so one wants to emphasize the complimentary compound adjective. Failing to hyphenate seems to miss that subtle nuance, but I wouldn’t consider it a mistake unless it violates some specific style guide.
But quite often these are real mistakes and real functional distinctions and not just academic pedantry. I occasionally encounter sentences where, due to lack of a critical piece of punctuation or some other small anomaly, I suddenly find myself thinking, “hey, that doesn’t make any sense”. I literally have to go back and read it again to infer the intended meaning. That’s an objective failure to write well.