A properly deployed Windows 8 isn't going to be any worse than Windows 7 - Windows 8 is more secure be design. As long as their software works, and as a hospital they would have tested the software within all margins of error, then there isn't a problem with it.
Everyone thinks "Modern UI"
Windows 8 isn't any more unstable than Windows 7. People will argue it is, but people argue about instability about all platforms currently available.
Proper testing and deployment is needed regardless of OS choice. I would presume that a hospital has done their research and testing to the best of their ability.