It depends on the range of the singer. Most women are mezzo-soprano's on average so they could probably handle her material easier. The songs on Dangerously In Love and I Am...Sasha Fierce technically aren't that hard to sing, the songs on B'Day and 4 are more difficult simply because they are sung at a higher register and people may not realize that. Usually songs in higher notes are seen to be more difficult. For instance parts of Green Light, Get Me Bodied and even Freakum Dress are very loud, aggressive and high those may not be easily replicated. Those require a strong belting/chest voice and a voice that can hit well into the fifth octave and stay there for runs and the like. Her money note F5's are also pretty high by comparison especially for an alto or just in general a singer who is used to singing lower.
I would classify her as moderately difficult compared to others. Not as difficult as Kelly Clarkson or Christina, not as impossible as Mariah Carey but still a tier above the likes of Gaga and others who sing lower then her and don't utilize belting excessively. Her songs are kind of like Carrie Underwood's.
Her work on "4" and "B'Day" are very difficult, simply because she sings with a distinct style.
Her work on "Dangerously In Love" and the "I Am..." side of "I Am... Sasha Fierce" are pretty simple. I dislike how she toned down her voice in "Dangerously In Love" to fit with the current trends. How even more amazing "Crazy In Love" would sound if she were to re-record that today.
I agree with everything you said, ESPECIALLY this.
When I was younger my vocals were more open, but now, I don't even TRY anymore.
If there was weight training for the throat, Mariah's songs would be the 100 lb dumbells.
Some of them, sure.
Some of them aren't difficult at all, like Brokenhearted Girl and all of the stuff on Dangerously In Love.
I think her most vocally challenging songs are on B'day and 4.
Most of B'day is centered at the upper 4th - lower 5th octave, and if you don't have that stamina and control those songs can easily tire you out. The only songs that aren't that high on B'day are Kitty Kat and Upgrade U.
Beyoncé birthed King B who conquered the immortal Sasha Fierce.