I'd say that the walnut is likely to be considerably softer than ash.
Ash no problem. Walnut probably no problem.
But. If you've ever seen a walnut tree (at least, an ABW tree, I'm not au fait with English Walnut as a specimen) you will know that nothing - I mean nothing whatsoever - will grow in it's territory. The area around the tree itself is totally barren. An ABW tree produces some sort of poison, there is no other word for it, which prevents anything else from growing.
Now I don't know it this something is produced by the roots only or by the whole tree. Even if the wood itself produces this, it may be an advantage in that it will prevent the growth of bacteria on the boards.
Hmm. I'm not at all sure I've added to the debate here, just provided more questions!
S