European ash can vary enormously in specific gravity. Some pieces can be as dense as hickory, while some are as dense as spruce. Accordingly, there is also a huge variation in the quality of the wood - the higher SG obviously being much better. The less earlywood and the more latewood, the better the piece. Some ash can be exceptional quality, while the majority is just 'nice' at best. The lower SG wood is so weak, that you need to design the bow as if the wood were spruce...
I'm pretty sure the same will account for American white ash, although maybe to a lesser extent. If possible (when buying a board for example) get a piece with fat rings, thin earlywood and a lot of latewood.