I have no doubt that she can sing, but I've yet to see her give a performance where she's singing in time to the music and has a real connection to what she's singing. I first noticed the lack of rhythm/timing on BGT. I thought it was something she'd get to grips with in time, then I saw a performance from her on TV this year (guest performance on BGT, I think...when she was singing a Madonna song) and she was still ignoring the tempo of the music and singing at her own speed.
I agree with the timing issues thing, Susan has a good voice, but that performance of You'll See by Madonna (I think it was that?) was horrible, her timing was all over the place or she just wasnt paying attention to the music. Other performances on BGT she sounded out of tune imo (maybe nerves, bad day at the office?)