I've tried both fuming and ebonizing, though on oak rather than walnut, since I am allergic to walnut. I much prefer fuming.
I found that ebonizing was far effective in 'blackening' the wood, so that the color became essentially uniform. On the other hand, fuming would darken the wood yet leave grain variation at least subtly visible.
I have used only household ammonia, mostly with white rather than red oak. It darkens white oak much more effectively than red. I have seen no need to use the >20% strength ammonia. In my experience, 12-24 hours exposure to household ammonia produces darkening to a depth of 1-2 mm (>1/16"). I have no interest in dealing with the stronger ammonia. Household ammonia is merely smelly and requires far fewer precautions, yet is entirely effective in my use.
The only caveat about my use of household ammonia -- I have only used it for fuming spindles and plates/platters, all of which fit in a 18x18x24 plastic tote bin. I have not used it for furniture. I do not apply liquid ammonia directly to the wood -- I depend on the fumes.