Here are a few pictures of underlayment as it is revealed in a phototype project I'm working.
Image one has a void in the central veneer of lowest of five layers of ply.
Image two has a void in the central veneer of the second lowest layer of ply.
Image three has a void in the central veneer of the top layer of ply.
Image four is the crack in the outer veneer layer that occurred when attempting to bend it to deck curvature.
I'm using underlayment because I'm not really concerned with the longevity of the boat. I'm developing a design and use it to meet other design objectives, but it does happen to give a valuable insight into the the construction of under layment material.
If you look closely (image one is the best for this), The top two layers of ply are 3 veneer ply. It is composed of a central thick core with thin surface veneers. This was the cause of the stress crack in the last picture. That and poor clamping procedure on my part.
The bottom three layers of ply are five veneer ply. The central core layers (3) are thick compared to the the suface veneers. Certainly, the five layer ply is going to exhibit better strength and build characteristics that the 3 layer ply. The surface veneers in either of these examples offer little in a structural sense and is there strictly to cover the inner cores.
The main point here though is that there is always the possiblility of voids in this type of a product. The voids as seen here aren't necessarily large, but they create discontinuities in stress distribution that could lead to component failures. You have to know what you are using.