I build my child themes on Twenty Twelve. Some of these other theme frameworks pique my interest, especially Underscores and Roots. I have heard a lot of good things about Genesis, but its usually from someone using affiliate links to Studio Press.
Can anyone name some major deal-changing advantages to using any of these or other options in place of the Twenty Twelve theme as a parent for a child theme?
Also, as an aside, can anyone explain why the Underscores site advises against using Underscores as a parent theme? It seems to me that for a theme that is so likely to be frequently changed and updated it would be all the wiser to use a child theme.