Arelith

I am not sure whether there was a stealthy change to weapons beyond what the updates thread shows, but it seems the light hammers' crit-multiplier ingame is only x2.
According to the Wiki it is supposed to be x3:

http://wiki.nwnarelith.com/Weapons#Weapon_Changes
Light Hammer size changed from Small to Tiny, critical threat range changed from x2 to x3.

Ingame the light hammer is showing it's base variation from NWN - neither tiny, nor at x3
This makes it possibly the very worst weapon in the martial-profiency featlist. Worse than even clubs which everyone can use without profiency investments.

https://ibb.co/bgS6Wxm

Have you checked your character sheet when equipping one? Sometimes these description texts are not up to date :)

It is x3, if you examine the sheet.

I can confirm this as someone running a light hammer WM build right now. It is a x3, goes to x4 for WM 5. And I can confirm that it is doing the damage it claims to be.

Though, now that the svirf hammer has been nerfed, I think your proficiency argument may hold true. It would be nice to see it added to a couple of other proficiencies, like rogue or even simple. I mean, I've seen enough zombie movies to know that anyone can pick up a hammer and swing it.