Well, actually, now that I'm a little more trained in music (in vocals, mind, and ears), I must admit I've increased in skill in singing Ayu's songs. I can sing most of them, unless it stays in the upper register for everrrr, or if I don't know the words. xD
Before I used to be able to only sing songs in the lower register, like... well, I actually can't think of any right now. XD
Actually, I sang alto in high school, and therefore stuck with some of Ayumi's lower songs. But now that I'm singing soprano more often, I actually sometimes struggle with lower songs like "criminal."
EITHER WAY.... I can sing almost anything, unless there's a large amount of jumps, a mess up of lyrics, or songs that stay in the upper register for measures on end. (Although, I admit I can sing "kanariya" without
too much trouble. XD)