I don't think Harris's 12th Symphony is that bad at all. It is somewhat uneven, however there is some fine imaginative writing in the work, it is just a pity he reused parts of the 8th Symphony earlier on in the piece. I agree that the 13th Symphony is poor, Harris was trying to draw attention to the US's less than perfect record on race up to the 1960's, and the suffering of black people in 'the land of the free', sadly with his mental faculties already failing, he just wasn't up to the task. The 11th Symphony I've long admired, though the two recordings I have, including the recent one on Albany, don't quite do the work justice (the Albany is short on strings and there are a couple of errors).
I've a number of of off-air tapes I would like to upload to this site when I get my tape-mp3 converter. A work like the 1st Piano Concerto might surprise some listeners, its light and rather jazzy, the Piano Trio and 1st String Quartet are fine works, and the String Quintet though perhaps over-ambitious is also very fine. There is also the setting of St Francis's Canticle of the Sun for coloratura soprano & chamber orchestra (from which some of the material of the 8th Symphony is taken) which is quite ecstatic. Harris wrote a number of lighter works, which I don't have and would love to hear and which should be recorded to show that he had a lighter side.
I suspected that this thread might attract your attention and, as a Harris expert, might give you cause for some annoyance :)
It is-of course-grossly unfair to judge Harris by the Symphony No.13 and I can recognise quality in the Symphony No.11. I am listening again to the Symphony No.12 as I type in an attempt to pick up on the "fine imaginative writing" you mention. It is, undoubtedly, not the disaster of Symphony No.13 but it does tend to meander a trifle and there is not the same driven sense of purpose and optimistic swagger of the earlier music. Harris wrote a huge amount of music-as you know full well-and the result is both a degree of unevenness and, I tend to find, an element of repetition which can exasperate me as a listener. The Harris-style is so self-evidently distinctive that I find it palling after several hearings. I can listen to Harris from time to time but I would not want to sit through a Harrisfest ;D
Harris was a fine composer and does not deserve his current neglect. Naxos appeared to be going to remedy that but-for whatever reason-chose the wrong conductor. Marin Alsop simply did not seem committed to the music (as she had been to that of Samuel Barber). JoAnn Falletta might have made a better choice. The only way to conduct Harris is to take the brakes off and go full-throttle-as a Bernstein could do in the 3rd.