I gave Eder (5th level) the war club and cleared out all the dungeons and most of the outside of  rederic's castle and he hasn't hobbled a single target. Is it working? I'm using the latest beta in case it matters

I tried giving it to other characters and none of them can hobble anything

Give it to a rogue, or a character with high accuracy who would be able to hit crit more often.

There are few chars that can crit as often as Edér with Disciplined Barrage.

 

Maybe the item is broken. Fulvano's Blunderbuss also doesn't charm in my game. And in one playthrough the Sabre of the Seas wouldn't proc Overbearing Wave (now it does).