Charles Shere writes about Copland's The Tender Land, here. I think Shere's characterization of the piece as "hokey and pretentious" is right on the money: the opera's music is thin and both libretto and score are unable, dramatically, to make anything particularly engaging out of its most potent theme, forbidden sexuality (which ought to work and which Copland handles so astonishingly well in Appalachian Spring, showing that the problem is compositional and not a direct function of either the materials or the style.)
Shere also writes that:
I've always thought of Copland, Britten, and Shostakovich as an interesting triad. Each was immensely gifted and intelligent; caught in an uneasy relationship to the prevailing Modernism-Reactionism duality of the early 20th century; apparently self-assigned to a position of National Spokesman for his art. Each composed masterpieces, particularly early masterpieces, then went on to an uneven output often troubled by indecision as to whether to be Popular or Principled (with respect to personal musical style).
It is, indeed, interesting how these three became "national spokesmen", as none of three came equipped with the biography or personality which could be described as a cookie cutter for such a role and the part of the local political and musical institutions in assigning these identities was perhaps more critical than their own contributions. While each lived through real political difficulties, on balance each showed a talent for adaption to their local systems.
Besides being the unavoidable composers during their lifetimes (as a kid, these were the three reliable exceptions to the rule of not-programming contemporary music on local radio and concerts), the members of this triad have received substantial attention of late from music historians with both scholarly (Taruskin) and more popular (Ross) projects. I'll have to admit to having large blind spots (deaf spots? why are we so persistently forced to use visual metaphors when talking about sound?) for the music of all three and this, collaterally, has made reading these histories a real chore. In Copland's case, for example, the music which lasts for me are the most "difficult" early and late pieces, The Organ Symphony or even the much-maligned Inscape, for example, with Appalachian Spring being the elegant exception that proves the rule for his "mainstream" pieces. With Britten, whose music I have really heard far too much of, the Church Parables remains impressive, and with Shostakovich, I have absolutely no patience left and now publicly pledge not to let myself get hauled once again to a new production of The Nose or Lady Macbeth of Mtsensk under the promise that "this time, you'll really like it". No I won't.
This is music which fails to renew the musical imagination. But then again, maybe, the official, mainstream status of these men and the stuff they made simply precludes any such ambition. It would have been best if we could've grouped the three into a Poe/Silliman category of a Musical School of Quietude, and let it all fade away. But it is in the nature of such boat-not-rockers that they continue to dominate concert programs and have a decisive influence on the young quietist and careerist composers of our time. And, unfortunately, in music, as opposed to poetry, our quietists are not restricted to the piano end of the dynamic range. There is one, small and slightly evil joy I can take, though: these are the three 20th century composers whose names are most likely to get mispelled on undergraduate music appreciation tests. Take your small pleasures wherever you can find them, I say.