Tasks that involve branching, for one. GPU's are fundamentally different kinds of processors. They are for applying similar transforms to vast amounts of data.

Tasks that involve branching, for one. GPU's are fundamentally different kinds of processors. They are for applying similar transforms to vast amounts of data. There are other area where big general purpose X64 or ARM cores excel too, like bit twiddling, memory random access, etc., but branching logic is the fundamental one.