I use them mostly for automotive, on gasket surfaces. They're great for removing leftover gasket material with minimal metal removal. Scotchbrite discs work too but there are applications where you don't want the fibers to get into where they can be harmful. The bristle discs are also good for light deburring & removing oxidation.
Yes the bristles are very flexible. The bristles are impregnated with ceramic, that's the abrasive. The white ones are 120 grit, that's the finest I believe. Scotchbrite discs I mostly use the maroon but also brown. These are more abrasive than the white bristle discs. I haven't tried any of the coarser grit bristle discs though (and don't plan on it) so not sure how those would compare.