Not the person you asked, but one game I had problems with on Steam that I did not on GOG was the OG Riven. It was still playable, but the various animations associated with pressing buttons and suchlike were completely broken. Very rare experience though and I have played many retro games on Steam.
GOG reportedly suffering from staff turnover and poor management: “Current business model is likely running out of steam” (gameworldobserver.com) angielski