I have to disagree. Harris' 5th, 6th, 7th, 8th, 9th & 11th Symphonies are all superb works. As is "Abraham Lincoln Walks at Midnight" and the Piano Quintet, 3rd String Quartet, Violin Sonata, "Give Me the Splendid Silent Sun" and others I cannot mention at first thought.
Strong, sinewy the ultimate in American experiences. Maybe you have to be born here. But I see and feel our vast forests, rivers, mountains as clear as I see Ireland in Bax or Finland in Sibelius.
I certainly hear the strength and sinew in Harris' music, and much more in the 3rd Symphony. It just doesn't feel as though he went much beyond it in his later works. I have to admit that I've never spent much time analyzing the other works, and various listenings haven't revealed any hidden profundity to me. I'll have to spend some time (goodness knows when, though!) on some of the works you mention and try to find more substance in them.