GT mounted them on the seat stays and they worked a lot better than chainstay mounting. A brake booster solves any flex problems. They are kind of a pain to set up though.
I like Biopace but have never tried Rapid Rise.
Lots of people moved them to the chainstay after they found out what a disaster the chainstay mount was. U-brakes kept me from buy a new mountain bike for about 5 years in the late 80s. I was so happy when the bicycle companies dumped them and went back to cantis.