I mean, face it. Whom is superfluous to requirements, and no one knows how to use it.
Well, maybe you do, if you were taught rigorous sentence parsing, or diagramming, in elementary school and remember all about it.
Now, I confess to a liking for whom. I do use it. But when I see it in print, it is used incorrectly more often than correctly. Take this example: She directed her complaint to whomever ate the pizza. That’s incorrect. Whomever is not the object of the preposition to; the object is the entire clause whomever ate the pizza, and whomever is the subject of the verb ate. So it should be to whoever ate the pizza.
While we’re on pronoun case (usage in the sentence determines case, that is, whether it’s I or me, she or her, they or them), it shouldn’t be to she who ate the pizza; it should be to her who ate… In this example, she her is the object of the preposition, and who ate the pizza is in apposition (an identifying or explanatory word or phrase) to her.* Again, it’s not her whom, because her is an object and who is a subject. And yes, it looks inconsistent to use her who, but it is correct.
Ogden Nash wrote a delightful little ditty, grammatically correct, about artichokes that starts out,
I seek in anonymity’s cloister
Not him who ate the first raw oyster…
Got it? No? That’s why we should just stick with who.
*A simpler example of an apposition is “the poet Burns…” per Merriam-Webster Unabridged online.