I guess it depends on your definition of challenging. For a performer in good vocal health, the score isn't really all that challenging, especially compared to some of SJB's previous roles (Wicked and even The Pirate Queen were much harder). I'd think it's harder to do what Allison does and land the comedy bits of the show than to just go out and sing, but that's just me. Allison literally carries the show on her shoulders, and she hasn't been out once. I remember hearing a story about how she was sick once and drank Pedialyte in the wings between scenes and just pushed through. It's really inspiring that in a day and age where people will call out over a hangover, there are people like Allison and Orfeh, and Andy Karl, and apparently Megan who hasn't missed a show yet either, who you can rely on and book your tickets any time and know you'll get to see them if you're a fan. Nothing against people who have poor immune systems, I know plenty of them myself, but it's unfortunate for fans of certain performers to miss them in shows sometimes because of attendance issues. We all know, me especially, that understudies can be just as good and sometimes better but no one can deny that it's disappointing to spend money and sometimes travel to see someone in a show and miss them!
I personally had a hard time catching Sutton in a few shows, but luckily I live in the city, so I can try again, not everyone has that luxury.