They are right. Sister Helen doesn't know, though she is learning. Moments like this make the opera truly dramatic. By any conceivable standards, Dead Man Walking is a big opera emotionally. That's part of what one instantly respects about it.
Many modern operas avoid emotionally big subjects in favour of attitude and obscurity.
There is not an ounce of those things here. Seriousness, on its own, may not be enough, but it is a very useful start. Heggie is an unabashed melodist, as he said himself at a symposium on the eve of the premiere. His music is rich and emotionally charged, betraying varied influences from Mussorgsky to Britten and Ravel, and carries enormous atmospheric power. The orchestral writing is full of interest and subtlety, and with Patrick Summers in the pit to bring the score to life, Heggie manages to build musical tension in a way that shows astonishing maturity for someone writing his first large- scale dramatic work.