Some sylables will sound easier than others, so even if the pitch is the same - the 2 notes may not be equally easy/hard to hit. A lot has to do with breath, placement, support, diction ..... it's a science, and it has limitations, that's for sure.
I can sing a "G" above the staff, but only in Piano Man ...... and only at a pretty good volume. In softer songs, with different words.... I stay away from anything higher than an Eb mostly.
Billy Joel and I both hate singing in "E".