AAQ – steinsaltz was called a kofer explicitly by the steipler and rav shach. No, they did not use that term mildly. The only place steinsaltz had refuge from the controversy was in chabad, because of an ancillary affiliation that he had with the lubavitcher rebbe. You will see steinsaltz gemaras in chabad shuls at times.
The issues with him can be clearly seen in his book “biblical images” where he casts people in tanach like characters in a soap opera. He also was part of a fake Sanhedrin. He was a very intelligent baal teshuva who had no mesorah and made up things as he went along. He also wrote garbled kabalah books in English for “laymen” and decided that he was qualified to reprint shas with his perush next to rashi’s.