Shockwave and Blaze

I'm guessing they are treating the knuckles similar to fomor weaponry and disable special upgrades to it due to it being a unique item. But it's not definite until someone upgrades their knuckles to see if it works or not.

You can add +7 max damage to the shyllien mana knuckles to probably go higher than a merlin max artisan knuckle if the focus is mainly fighter skills with magic as secondary.

Meaning lack of int magic unless bolts are used, or unless you reforge to get bolt casting speeds increase.

EDIT: You can also pair cylinders and use sand blast alongside with blaze if you want to make it viable. Sand blast is very useful in dangerous situations when you gotta stop one enemy from aggroing.