Why would an ebony fb be overkill when a phenolic fb isn't
Overkill because of the expense of ebony. Phenolic is also probably more durable than a fretted fb reasonanly needs to be, but it is cheap, so it is less of an issue.
There really is a tonal characteristic factor to ebony
I don't disagree. But, I do believe that the tonal character of any fretted fingerboard is minimal. In the case of ebony, I might argue that the overall tone effect caused by ebony is somewhat greater only because it adds more overall rigidity to the neck than other woods might. Still, you could create the same rigidity with more cost-effective methods.
I don't mean to argue that ebony is a poor choice for a fingerboard. It is an excellent choice. I am just offering possible reasons why it is not commonly offered.

