Here's another possibility - what if exotic weapons and weapons are not in the same category, and when the mood starts it is already decided if you will be getting an exotic weapon OR a normal one?
If that's the case, you can scum your moody dwarf's creation as often as you like, you'll have 0% chance of seeing one of those exotics from it.
This mod is heralded to be incredibly different. Quotes from the first post: "Countless hidden features and easter eggs which, individually, will not affect gameplay 99 out of 100 times, but are there to be discovered. No playthrough will be the same!" and "When you get a weaponsmith in a strange mood with my mod (or in the unlockable "weapons concept laboratory"), you have no idea what you're going to get. Maybe you'll get a boring old sword or axe, or maybe you'll get "Blacktemples, a mithril khopesh," or "Hauntedrazor, a damascus steel pair of bladed chains." Legendary weapons should be legendary. Your civilization will be known forever for the weapon it creates, which will truly be one-of-a-kind. "
How rare does something have to be, for our (usually very accurate and honest) Narhiril to state that such things "will truly be one-of-a-kind"? I say, very rare indeed. Play/playtest all you want Sir; our modder has promised us a game which will never play the same twice.... and for that matter, this has so far proven true for you too, has it not?
This is, in my opinion, the VERY greatest of the Easter Egg mods (and the very greatest of all other mod categories, but I've more proof to support the Easter Egg category than the others).
Another detail I'd like to comment on, you mentioned running into some disappointing voidwalkers.
I've met some wimpy voidwalkers too. And plenty that were not. Usually intermixed within the same game. There's nothing to say that this one playthrough you are doing now might even have a civilization of voidwalkers who are almost all wimps - that certainly would be a unique happening that I know of. Then again, voidwalkers have their own type of very special projectile weapon, and impressive aim at a distance, even through fortifications... Maybe your game just needs a little more time for them dice to keep rolling.
I expect that THIS mod is going to be very hard for you to accurately define the odds on. It's designed to be very variable; not just across one game but across many games. I suspect some of the random stuff gets decided during world generation, and that there are scads of surprises that absolutely cannot possibly happen in any specific game that is already being played, because the variables that allow for them have already been selected against - that's one very nice way to guarantee that no playthrough will -ever- be the same, after all.
So keep writing of your adventures, and your satisfied or frustrated desires to delimitate and define what you see happening and what you think that means for every other playthrough too - I'd only advise that you perhaps phrase your findings either within the framework of a truly impressive pile of varied experiments that test what you believe you see, or that you speak of them as 'this is what's happening in my game', without adding in 'thus it's what happens in this mod for everyone'. That's likely to keep those of us with teeth and troll-thirst benign, if not benevolent, as we consider your words and decide how to answer you.