Hand Tools Archive
Depends on the ply.
The high-quality "multi-ply" ones typically have 1/32" face veneers, so they plane quite well. They're no more or less difficult than a glued-up top, and often better depending on how the veneers are matched on the show face. Of course that stuff isn't cheap. I think I paid a bit over $100 per 4x8 sheet of 3/4" multi-ply Maple at my local dealer (MacBeath in Berkeley).
The more common "veneer core" stuff is all over the place, with some of the cheaper examples having paper-thin face veneers and relatively soft core veneers underneath. They're more difficult to plane both because you don't have much margin for error, and because the more "springy" (softwood or poplar) core underneath gives the surface somewhat unusual and tricky stiffness properties.