I'd agree that marine ply is way overkill but caution that "engineered materials" vary widely in their properties. I've seen some 15-20 year old kitchen cabinets starting to crumble next to a dishwasher and sink, where heat and humidity are highest. The cabinet doors are simply pulling off the
hinges, the material they are set in is falling apart. And, every door in the cabinets is out of kilter as they shifted in the screw holes.
But a lot of good commercial cabinetry is built from engineered materials, which I still lump together as "particle board". (I don't like it.) A recent "flip that home" show on TV pointed out another potential land mine: The new owners wanted to install granite counter tops, the counter people said no way, you can't put that on composite boards they'll buckle from the weight.
If you are building the lower units...and there's any chance you'll want to place a heavy countertop on them, don't use composite. And if you do use composite, check the particular materials, and maybe see for yourself how well they hold screws or resist moisture, try a sheet before you commit to a whole project.