When to use ‘who’ and when to use ‘whom’ seems to be one of the most common areas of confusion for English learners, and even possibly for native speakers. Personally, I don't find it confusing at all and (although I am no grammarian, as reflected in my answer) I even tried to reply to one of those questions in April 2017, my first month at ELU:
I later found that members regularly and repeatedly ask about ‘who’ and ‘whom’ here:
‘Who and whom’ questions also get asked at other grammar websites with great regularity.
So linguistically speaking, what is it about ‘who’ and ‘whom’ that is so difficult for so many new learners?
Non-native speakers trying to improve their English often tend to go by rules rather than usage. Could it be that the 'rule' covering the use of who and whom is itself complex, ambiguous or contradictory?
Note: I am not asking what is the difference between who and whom, so somebody please don't close this question as a duplicate unless someone has previously asked specifically why who & whom create such difficulty for so many learners.
Nor is it primarily opinion-based if you can quote standard references or expert commentators to support your answer.