I think training and self care has a lot to do with being able to sing a role properly and for a long run.
Patti admits in her bio that she didn't know how to sing when she played Eva and was trying to learn on the fly. I think singing Eva is do-able when you have the skill, the power and the discipline.
I think it is harder to sing rock scores, so I think that Diana for N2N is among the more difficult ones. However, Marin was much more skilled than Alice at vocal care and technique and never burned her voice out and didn't miss shows.
Alice blew out her voice and then would be caterwauling on youtube and the like. She really seems to have to no sense of vocal care.
Jackie Burns, one of the most consistent Elphabas, takes extremely good care of her voice including not talking much during her off hours. She takes the role and delivering the songs seriously.
"The sexual energy between the mother and son really concerns me!"-random woman behind me at Next to Normal
"I want to meet him after and bang him!"-random woman who exposed her breasts at Rock of Ages, referring to James Carpinello