Taken by a client to one of Britney's performances, I'm not sure how to enumerate the faults, other than to assign a rating of "over-blown mediocrity". Lip-syncing is bad enough, and even worse since she doesn't sing well.
All I can say is that I mourn the years since seeing Ethel Merman and Louie Armstrong on the same Vegas stage, after seeing Roger Williams playing in the lounge.