I don't know if I would consider "Kyuss" to be "unsuccessful" by any stretch of the imagination. There are now hundreds, perhaps thousands of imitation bands out there today, because they liked their sound so much. They may not have initially sold billions of albums and became uber rich, but they kinda crafted their own subgenre of stoner rock, and they have a huge following. At the very least, I would consider them to be Successful.
I agree with Browntown and GagHalfrunt about Blue Oyster Cult. They may not have been "super megastar" blablabla, but they are very successful.