From the “First Superstring Revolution” on, there have always been skeptics, even though they often were not very vocal. Perhaps the most well-known piece of such criticism was Paul Ginsparg and Sheldon Glashow’s Desperately Seeking Superstrings, which appeared in the May 1986 issue of Physics Today. I recently became aware of some other similarly critical articles by Noboru Nakanishi, and copies of them have been made available to me. They are:
Comments on the Superstring Syndrome (also from May 1986)
“Superstring Theory” Syndrome (published in the popular magazine “Parity”, September 1986)
Can the superstring theory become physics? (January 1993)
This last paper claims that “the bubble of superstring theory has … bursted”, which, in 1993, was rather premature.