I agree that a lot of Guettel's work sounds similar -- but that is just his style. Most composers works sound similar in style to their other works. There are certain chord progressions, cadences, intervals, melody patterns, rhythms etc. that each composer is fond of and they use them in their works. That is what makes them unique. Just by listening to the scores without prior knowledge I could identify that
Myths and Hymns,
Floyd Collins, and
Piazza were all by the same composer. By that same token I could pick out a show by Sondheim, Webber, Yazbeck, LaChiusa, or Jason Robert Brown without prior knowledge too. They all do things in their music that is distinctive of their style of composition. It's just like listening to a classical music radio station and knowing immediately if a piece is by Beethoven -- his style is very recognizable.
Of course there are always exceptions here and there. If you listened to some of Beethoven's later works, it might be hard to pinpoint it as Beethoven because by then his style had moved from Classical into what would later be deemed Romantic. If all you had to listen to by Guettel was "The Ballad of Floyd Collins" and "Aiutami" then it might be hard to guess they were by the same person -- but if you listened to the entire scores of both shows it would be obvious. When I play the song "Lonely Room" for people who are only familar with the movie of
Oklahoma! they never guess it was composed by Richard Rodgers. However, obviously the rest of the show is in the classic Rodgers style.
Finally, I see where your voice teacher is coming from. Some of Guettel's work is very challenging and doesn't fit the vocal line well, but it is not unsingable. It is just very, very difficult. However, songs like "Migratory V", "Hero and Leander", "The Light in the Piazza", and "Passeggiata" all fit very smoothly into the voice. Sometimes I wish my college voice professor were more like yours -- he knew that I was comfortable singing musical theatre and melody driven pieces so he always wanted to challenge me by having me sing twentieth century art songs. Which I guess in the long run probably was better. My last year though, which was 1999, I was able to sing a selection by Jason Robert Brown and one by Guettel on my senior recital. The voice faculty was very impressed by the musical vocabulary used by these two composers. Up until then the only musical theatre composer whose works they thought were challenging enough to merit us occasionally singing them at recitals was Sondheim.
"You pile up enough tomorrows, and you'll find you are left with nothing but a lot of empty yesterdays. I don't know about you, but I'd like to make today worth remembering." --Harold Hill from The Music Man