the heart wood of yew is harder than ash, and I don't see anybody complaing about that being too soft?
Use it and see how it works! I've got an axe with birch on the handle,another with oak and yet another in hazel. none have broken yet
ash and hickory are best, but basically anything will work, it's just how long it will work for. Even St Mears was on telly last night making an axe handle from poplar, and that is like loo roll!
http://www.oetzi.com/en/axewell, the famous axe of oetzi the icemummy was yew as well- a section of trunk with a sidebranch sticking out at ca. 90degress if i remember correctly
Great for bows. But it is a soft wood which may not be good for an axe handle.
Yew is indeed a softwood - its a gymnosperm. Balsa is a hardwood though (angiosperm)