It's interesting. I saw the show in New York, and while the girl I went with wasn't a huge fan, I found it okay. When I saw the album came out, I gave it a listen, as I had also checked out the Mean Girls album and was pleasantly surprised by all the nuance I missed in that score. With A Letter to Harvey Milk, it's kind of the opposite effect. The music seems more bland than when I heard it in the show.
The orchestrations are well done (they had a John Tunick-esque sophistication), the cast was very strong, and I'm sure the writers had the best intentions, but the show just feels like a message delivered too late. The moral is about accepting LGBT people for who they are. That message was probably a lot stronger in 2011 when the show was being workshopped. But this is 2018, almost 3 years removed from Obergefell v. Hodge. While there is still a long ways to go, we are mostly past the point of "gay people deserve basic human respect". Shows like Hairspray or Ragtime can take civil rights issues from the past and make them feel fresh and relevant and important, but Letter to Harvey Milk doesn't seem to accomplish the same feat.