I still have some 40 year old 4'x2'sheets of 9mm Sundeala in my loft which are still flat, and I never had any problems with baseboards I made with it. They were leftovers from a job I was on and therefore free. For its price now, £55.00 for an 8'x4' sheet locally, I would sooner use Baltic Birch.
What I can't find now is the 8'x4' sheets of 12mm Woodfibre Insulation Board, which was £8.00 a sheet last time I bought some about 5 years ago.