Thanks for taking the time to respond, Tom. Your view is most welcome. Seriously, in matters of the arts I'm disinterested in unified opinion. Differences of opinion go hand in hand with the arts; I'm not only comfortable with that but often happy about it - if a work of art doesn't elicit diverse opinions then it's failing to reflect the essential nature of human existence.
Be that as it may, I'm not convinced this film managed to produce much art at all - in terms of the screenplay and the score as a supposed artistically unified entity. I'm not concerned with either of them in isolation but both together, in that film. Scoring to film is (at least should be) an art in its own right. These two main ingredients should ideally be inextricably married, and my main criticism is the sheer amount of non-connection or incoherence between them in Mank. As one huge consequence, I was unable to engage affectually-emotionally with the film - and I'm certainly not interested in watching any film merely for its intellectual content. (To address your concern as well as I could, Tom, I went back and endured the film up to about 1hr 15mins.)
In fairness, one could say that any film score composer would have had serious difficulty with the awful lack of affectual-emotional fluency and coherence in this film's dialogue and, most importantly, in what I guess is supposed to be the storyline. But isn't that exactly the kind of situation in which real scoring talent is most desperately needed? Reznor & Ross clearly weren't up to the task of at least mitigating if not ameliorating that situation, let alone fixing it. What we're presented with, most of the time, is the dialogue/storyline on the one hand, and the score on the other, both going in their own directions with an appalling degree of disconnect between the two.
Spotting was also amateurish in too many places, especially in the early part of the film where the score was overly present. Spotting is an art in itself but here again the lack of tact, intuitive delicacy and emotional coherence rears its ugly head. I've no idea who is to blame for that.
Reznor & Ross seem to have divided their efforts mainly into two categories of music - atonal and a bit weird, anomic, even schizo, on the one hand, and '40s swing/jazz band on the other. It seems they intended the atonal stuff for narrative purposes, and the swing/jazz for period-authenticity in scene setting. Perhaps my biggest complaint is that they failed miserably in trying to meld their atonal music with the screenplay.
I've never been much of a fan of atonal music, and the Mank score simply reinforced my opinion that ET-centric atonality is extremely difficult to pull off such that it stands as genuine art, rather than a crafty evasion of art. (Goldsmith and Horner both pulled it off nicely in the Alien franchise; a rare feat.) There are a few moments in the Mank score where clearly an atonal passage comes off the rails; it may have passed as ok on piano (or perhaps electric guitar) but really should never have been given to orchestral instruments to play - in some places it just sounded out of tune or merely faulty harmony, through no fault of the instrumentalists.
I'm totally fine with swing/jazz band stuff as such (my absolute fav being bebop) - I recall always with great pleasure that wonderful score in Billy Wilder's masterly film, Some Like It Hot. Well, I could say that in Mank the swing and jazz were a bit too much on the tepid, anaemic, simplistic side, despite being played well by skilled musicians who clearly knew what they were doing; but that's by the way. My main complaint is that it was simply detached, incidental music; nothing to do with the dialogue or storyline one is supposed to be engaged with and following on screen. It's somewhat like the old habit of TV soaps in having a radio playing contemporary pop in the background, just for scene-setting purposes. Really, for purposes of period-authenticity Fincher could have done much better by adding a few excerpts of genuine classic swing/jazz tracks at apposite moments.
All in all, Mank is said to be a hit job on '40s Hollywood. An extremely tough and tricky assignment, yes, and I'd have enjoyed it if it worked to that end. But Netflix shot themselves in the foot with Mank - I don't suppose I'm entirely alone in having found it unwatchable.