Hi John

Well, if you mean to say the heirloom in the sense built as they use
to build it - then yes, using a plywood is a no no. If that's not
what you mean, plywood - typically veneer plywood - can be the better
wood choice to make. Plywood doesn't change its dimension as much as
real wood (being made up of wood in different orientations - the
expansion/contractions kind of cancel out)- is more stable (since you
would probably have to glue up pieces to make the sizable panels) -
and per board ft, probably cost less. So, you can make a better piece
of furnitue, that will probably outlive you, and become the heirloom
piece in your family.

I just made a coffe table, and I choosed a glue'd up oak for the top,
pretty much veeneer plywood for the rest. Even, as I made it, the top
was warping a little bit, while the plywood was as flat as can be. I
edged the plywood with oak strips, and I think it looks pretty good.

I didn't do any of that fancy "floating" joinery where you let the
wood more or less float on top of the rest of the table - using "break
board" edges and oblong screwholes to let the wood expand. I'm half
expecting to see that top - pop off one of these days as its only
dadoe and glued to the rest of the table. So, well see how good the
"real wood" does.

At my level of comfort and expertise, I stick with solid wood for the
small parts or "show" part and plywood for the rest.