That Betty Buckley is notoriously difficult is fairly common knowledge within the biz that we call 'Show'. A stage manager I know who likes actors called her "a miserable human being", and that is a quote.
That said, my problems with her as Mrs. Lovett are twofold. She has a huge metallic-sounding voice that seems out of place in Sweeney Todd. It is a voice that can nearly etch glass and is not always a good sound. Bluntly, she shrieks and lacks warmth.
And (he ventured gingerly) isn't she...well, a trifle mature to play Mrs. Lovett next to Shuler Hensley? She's 60 if she's a day.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher