Well I've done several tests with LANDR to try and reverse engineer what it does, mostly by trying to fool it

Long before there was any discussion at Cakewalk about LANDR, I wrote a column about online mastering for Pro Sound News (IIRC correctly it was in September). I basically detailed what I thought were the pros and cons, which over time, have been shown to be correct.
Probably the test of greatest interest to you was I mixed two files, one with too much emphasis at around 400 Hz, and one with too much emphasis at around 1.5 k, then fed them both into LANDR to see what would happen. LANDR reduced the 400 Hz in the first file and the 1.5 kHz in the second file. It was not reduced as much as I would have reduced it, but I assume that's because LANDR thought the peaks must have been mixed up that high for a reason, whereas I would have just assumed someone didn't know how to mix

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One element of mastering that has been consistently overlooked is there is not one and only one "correct" way to master a song. All mastering engineers make value judgments. The best example I can give is of a mastering job where I got the files just before the client went off to Italy. Normally I do a lot of back-and-forth with clients, basically interviewing them to find out what their vision of the music is so I can enhance it as much as possible. But I didn't have that opportunity because she was out of contact.
The song could have been mastered as a more "chill" piece by pulling back on the kick a kit a bit and bringing down the highs where the percussion lived. Or, it could have been mastered as more of a club mix by bringing up the kick a bit, especially the beater, highlighting the high end a bit more, and speeding up the tempo by 2%. I had no idea which one she wanted, so I did two versions. When she came back, she said it hadn't occurred to her it could be done both ways...she ended up using them both, one as an alternate mix.