You could go ahead quoting Wikipedia:

UCLA historian J. Arch Getty wrote of Solzhenitsyn’s methodology that “such documentation is methodologically unacceptable in other fields of history” and that "the work is of limited value to the serious student of the 1930s for it provides no important new information or original analytical framework. Gabor Rittersporn shared Getty’s criticism, saying that “he is inclined to give priority to vague reminiscences and hearsay … [and] inevitably [leads] towards selective bias”, adding that “one might dwell at length on the inaccuracies discernible in Solzhenitsyn’s work”. Vadim Rogovin writes of the eyewitness accounts that Solzhenitsyn had read, saying he “took plenty of license in outlining their contents and interpreting them”. Both Rogovin and Walter Laquer argue that the book belongs to the genre of ‘oral history’.