I agree and disagree!
Manufactured retail product, (unless built for commercial usage, full sized aircraft, Man carrying), are built to a price. Just because a design is rendered in foam, balsa, Plywood, plastic, FGlass or even CF does not mean that it was built with the absolute best material. Sure plywood has great rigidity but also great brittleness combined with long term delamination issues.
As structural loads do not always act in the same direction throughout a model wing or fuselage and if the parts are cut via laser or similar there will be some areas that are not optimized for proper use of the material strengths and so extra material is normally used,
Personally I have FB, Foam, foamcore, Balsa, and plywood planes most factory built, (not the FB of course), and each has its own strengths and weaknesses. Additionally I repair models, (any model), for other club members who have problems in effecting their own repairs, construction materials not an issue!
Apart from the materials there is the construction methodology which can make or break the plane. I am sure that if you took a balsa design and cut the parts out of Ply then it would be way too heavy to fly well and conversely if you took a plywood design and built it out of Balsa it would be weak or even require serious amounts of additional material to make it fly properly. The design is the Key! Look at the plans for 2 identical planes made in different materials and the required differences and the different approaches will be very obvious.
There is no comparison of all ply Vs balsa Vs some other material you can build in whatever you like or choose. Just remember that each material has its place in a properly engineered Model.
Have fun!