It's not the range that's a problem for me. I can do baritone, tenor, alto... and when a guy sings the same notated pitch, it's an octave down from the female. So a C in the staff for a tenor produces the same note as an Alto's C under the staff.
The part that makes it hard to sing is that the female singers have a certain quality to their voice, perhaps a husky popish quality... that makes it hard to do. I have no problems with the Aria from FF6, but Eyes and Melodies are a pain to do...