ThomD
>Good question, I don't know the answer, it would depend on specifics, particularly your local/in house climate. I did make some kitchen cabinets once in the deep dark past that consisted of two stiles on the side, and a plywood panel mounted in the grooves in the stile. The surface was scored in a geometric pattern, not my design. That worked because the frame didn't encircle the panel, but it probably would have worked if the panel had been encircled since plywood moves very little at all. It's movement is more in the order of swelling, and that requires a fair amount of moisture. Many kitchen cabinet use glued-in ply panels and those coped corner joints right off the router that do not have much MT type engagement. But even there there is the minute movement in rails and styles themselves to deal with.

The whole point to frame and panel is that you can eliminate the guesswork, by floating the panel. Pocket screws, while cheap looking, have the reputation of being very strong, so I would not hesitate to use them in an actual panel construction. Another option that is just as fast are biscuits. They probably won't last out the centuries, but are very strong for as long as the glue joint lasts. The main failure mode is the break it over your knee type mode, not the hanging from the hinge mode.

When working out of the reach of decent materials, I have made some very handsome doors from mahogany jam sets for the frames, and bead board for the panels.