Several bishops denounced Newman to Rome as a heretic, and the very orthodox Cardinal Manninger certainly considered him to be one.
St. Pius X, unfortunately, made the mistake of backing Newman ... undoubtedly due to his advisors, since he obviously did not have the time to read the torrent of writing from Newman.
Newman had been very much opposed to Vatican I, and told those who opposed it after the Council to lay low because some day the "interpretation" of Vatican I, of papal infallibility, would change. He clearly had a Modernist view regarding the "development of doctrine" (despite people who tried to claim otherwise), and Newman actually was the one who allowed the Modernists to get that initial foothold into the Church.