Some cover artists prefer to use traditional media, oil paints especially. This is true for many contributors to the Muddy Colors group blog managed by oil painting artist Dan Dos Santos.
A somewhat older artist than the Muddy Colors crew is British cover artist John Harris (1948 - ), who worked in acrylics and other media for some time, but finally settled on oil paints because he could best achieve desired effects in that way. The best biography of Harris that I could find on the Internet is here.
Harris can be painterly or (comparatively) hard-edge, depending on requirements. Here are examples of his work.