Complex sentence: The group never gained any mainstream audience, but were an influence on their contemporaries--most notably Soundgarden, The Melvins, and Green River--alongside whom they are considered the early pioneers of the sound that would later be called grunge.
Split into simpler sentences: The group never gained any mainstream audience, but they have been an influence on their contemporaries (Soundgarden, The Melvins, and Green River). These bands are considered the early pioneers of the sound that would later be called grunge.