Keep in mind that if repeating crossbows were any close to being as effective as a regular crossbow, the Empire army (and many other nations) would be using them as standard weapons for their troops. This is, obviously, not the case.
Simply put, the repeating crossbow has a couple advantages over the regular crossbow, but not enough to mkae it a superior weapon. This is how it *should* be.
Please keep in mind that WFRP is not about making every career and every piece of equipment balanced with every other one. It is a story, and as much as anything, being "cool" or "unique" is as much of a benefit as stats. Hence, for example, black powder weapons are for the most part inferior to bows and crossbows while costing more.
I get what you are saying about not everything has to be "balanced" in an RPG. This isn't a board game with competition, its about telling a fun story together.
However, I do believe that RPGs also involve choices. And choices should be fun, with interesting options and tradeoffs on all sides. Making these choices and deciding which tradeoffs to make is part of what makes each character unique. Thus things should at least be somewhat close to being balanced, otherwise fun choices are reduced to "Well do you want to be effective, or ineffective?". Repeater crossbows as they are written are neither close to being balanced, nor a fun option. They suck. They suck really, really badly. In fact, the only ranged weapon you could say is actually worse is the Improvised or maybe the Whip (though at least that has the fun side effect of being Entangling). Making it even more odd, they are one of the most expensive weapons in the game, costing a whopping 3 Gold (a fortune by any standard in Warhammer).
3 Gold is something characters would scrimp and save for, over long periods and many sessions in most games. And their final reward is a craptastic weapon that they have to spend 2 turns reloading once it expends its 10 shots?!?
You say that "the repeating crossbow has a couple of advantages over the regular crossbow". Actually it has one advantage. It takes 4 manuvers to load 10 shots instead of 10 manuvers. That is its only advantage. Everything else is a detriment, damage, range, and cost (and I would also argue reliablity though that isn't spelled out by the rules).
My point is, I'm not trying to obsess about having everything "perfectly balanced", but I do want Repeating Crossbows to be a fun option. Something players would consider as part of an interesting choice, not feel punished if they decide they want their character to use the "new-fangled crossbow".
Also I would completely disagree with your assesment of black powder weapons being inferior to bows and crossbows while costing more. In fact it is quite the oppostie, they are superior to both bows and crossbows; while costing more and having the Unreliable quality. Once again, fun choices and interesting tradeoffs. I want Repeating Crossbows to be another fun option with interesting tradeoffs.
Also note that despite Black Powder weapons being superior to both bows and crossbows, they aren't the "standard" issue to the Empire's ranged troops. This is due to their cost and unreliable nature. I believe the same would hold true of repeating crossbows, even if they were clearly superior to ordinary ones.