If you use lumber all the wood grain is going lengthwise and will contribute stiffness. With plywood half the laminates are oriented crosswise and would be just filler in that application.
Lumber will also be easier to plane or otherwise work if that is required and will be a better recipient of metal fasteners if those are required.
So, would it be a problem? No, but there are better options.

: I'm using a combo of shearclamp and tape on my current build. Sheer clamps in
: storage areas for and aft. Can anybody forsee a problem with using 12mm
: ply for a sheer clamp? the outside seam will be taped.
: cheers, Craig