I believe, Feinberg was one of the greatest minds and personalities in the 20th century music. Although not nearly as famous as he deserves, this extremely modest man (to whom any sense of self promotion was completely foreign) for me is one of the finest examples of the personal and artistic integrity.
As a pianist for me he stands in the same league as Rachmaninov and Hoffmann. His thinking about music (and life) was monumental and global--he never learned "pieces", but entire volumes of pianistic literature--he had in his repertoire entire clavier Bach, Beethoven, Chopin, Schumann, Schumann, Scriabin, Shostakowitch... His musical intellect was simply formidable--in one concert season he could toss a few series of concerts including Beethoven 32 Sonatas, Bach entire WTK, and Scriabin all 10 Sonatas.
As a publicist besides many others he wrote a monumental work "Pianism as an Art", which in my view so far is the best essay on the topic, ever written.
In 1927 in one of Dutch magazines there was published an article "Feinberg or Stravinsky?" discussing who would pioneer new ways of music development.
While there can be seen influences and some similarities with Scriabin, I believe Feinberg's style is completely unique in many ways, and in his work he went quite a bit further than Scriabin.
Best, M