Both, basically. To get good results on a complex scene you need to increase the subdivision count of the radiosity calculation by a lot, which will be slow un

Both, basically. To get good results on a complex scene you need to increase the subdivision count of the radiosity calculation by a lot, which will be slow unless you're smart about it. And radiosity only accounts for diffuse bounces, leaving out glossy and specular reflections. The Wikipedia page has a section on Limitations.