I've overwintered colonies in 18mm ply nucs.
The only problem I find with them is that unless you protect with a decent exterior paint or something like cuprinol colours the ply deteriorates very quickly, even high grade exterior ply (multiple thin sheets layered is better than a few thick ones used one the cheaper varieties).
I make all my own nucs (I run Langstroth) but have also made 14x12 so I can convert if I need to, and use the 14x12 nucs for collecting swarms. All are painted (2 coats) with Cuprinol Colours(slate grey).
