The reason you see lots of ash flatbows is that ash is well-suited to that design. I've seen some ash longbows made by mostly European guys, but I think the European ash may be a bit different from ours. Our ashes are pretty variable, with white ash being much better bow wood than green, black, blue, or other species. Ash makes a good flattish-bellied bend-through-the-handle-longbow. I don't know how it would stand up to a stacked cross section. If you have plenty, try it and see.