Personally I thought Aoiffe was the better actress on the HDYSAPLM shows, and thought back then the perfect role for her would be the Mother in Blood Brothers. I wish Bill Kenwright would put her in that show!
Re the 8 performances a week. I appreciate it's very disappointing if you book especially to see a specific performer, as I did a few years ago to see Sarah Wildor in Contact, and just before curtain up, it's announced that her role is to be played by the understudy.

When I rebooked, and did see Sarah, the difference in quality was staggering. That was one understudy who was out of her depth, but on other occasions, it's been a joyous experience.

I saw Antony Andrews 3 times as Professor Higgins in My Fair Lady, and Harry Burton once, on a midweek Matinee, who actually had a far better singing voice, but lacked the charisma and stage presence of Andrews, but nonetheless it was enjoyable to see the understudy. I also spoke to other audience members who'd seen the Eliza understudy, and said she was wonderful.
But I do agree that if you are in a demanding role then 8 performances a week is tough.
Francesca was on LBC Radio last Saturday commenting on the whole Nancy experience, and working in Musical Theatre, and she said when she was in Rent, she'd wake up every day unable to speak....then as the day wore on and her voice loosened up she knew she'd be okay to perform that night. But she said when you're in a run like that, you can't do anything else but gear your entire life around your performance, so I'm sure that takes it's toll.