https://www.amazon.com/Arrau-Music-Performance-Dover-Books/dp/0486408469An extract from this book:
"... I thought the sound [Rachmaninoff's] was not very good. And from the standpoint of interpretation, it was appalling. He [Rachmaninoff] didn't seem to care at all what the composer meant."
Lastly, the author states: "though he [Arrau] regards Rachmaninoff as a shallow composer".
No real reason as to why he thought this though. Music is a personal "thing" after all.
In the book 'Conversations With Arrau' by Joseph Horowitz, Claudio Arrau says this :
"Rachmaninoff was a really great pianist, but not a great interpreter, because he made everything into Rachmaninoff. He was a sensation in Berlin after the First World War. I heard a few recitals - it must have been in the twenties. Technically,he was phenomenal. But I thought the sound was not very good. And from the standpoint of interpretation, it was appalling. He didn't seem to care at all what the composer meant. He even added several bars of his own to the end of the 'Funeral March' Sonata of Chopin. You know, once I played the Beethoven 'Eroica' Variations in Chicago and Rachmaninoff came backstage during the intermission to tell me how beautiful it was. He had never heard of the piece before. He was very friendly, very complimentary. But he wasn't even surprised that he had never heard of it! The 'Eroica' Variations!"
It is interesting that Arrau played Rachmaninoff's Third Piano Concerto frequently between the two world wars. That is interesting because, to me, Arrau and Rachmaninoff are the biggest mismatch in the history of piano playing. During that time period, he was often forced to play pieces he did not like because otherwise he would have to starve to death. Today, his easiest-to-obtain recordings are those he recorded for Philips from the 1960s to the 90s. Many of his earlier recordings, which have been re-issued on CD by Arlecchino, Dante, EMI, Pearl, RCA etc., are considerably harder to find.