Complex sentence: Their success in Japan was huge, and according to some critics, the band could be in the list of the 100 main artists of all time in Japan; a great achievement for a group whose songs were considered to be outside traditional Japanese quality standards.
Split into simpler sentences: Their success in Japan was huge, and according to some critics, the band could be in the list of the 100 top artists of all time in Japan. This is a great achievement for a group whose songs were considered to be outside traditional Japanese quality standards.