Crossbows in general are already incredibly front-heavy weapons.
The magazine would generally be placed around the mid-point of the weapon, if not behind.
Thus, it would not likely be a meaningful negative contributor to the weapon's balance.

It's definitely not worth an exotic weapon feat. I could just about see martial. Even then, it's still not as good as a longbow. It does (as a heavy crossbow) one more point of damage, which isn't worth having to take an action to reload every five shots. The only possible advantage is being able to use two light crossbows at once, and that's still not all that great, especially since you're going to have a lot of trouble reloading them with any speed (even if your DM allows you to apply rapid reload to a repeating crossbow). As a martial weapon, they're still somewhat worthwhile for a fighter-type who wants to have a back-up ranged weapon.

I think this is one of the main factors.
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Already front-heavy, the clip seems to make the crossbow even more unwieldy and difficult to hold steady to aim / fire.

It might also be 'exotic' because it's an oriental weapon, where D&D is primarily western-culture.

No, just making referential humor over a recurring topic.

I considered that, but weight and balance rarely seems to affect the ability to wield weapons elsewhere (unless you count the warmace, which imposes a -1 penalty for how heavy it is)

Bastard sword, 1-handed
Dwarven Waraxe, 1-handed

Which brings us to the inconsistency argument; A warmace has the same reason stated for its disadvantage, but nothing overcomes the additional penalty inherent in wielding one (turns out its an Armour class penalty, what?) And a maul weighs twice as much but doesn't have this same disadvantage.

A weight/balance/size issue doesn't make sense when its only selectively applied, sometimes even within the same book.

If a repeating crossbow is anything like my old crank loaded rubberband rifle, then I can tell you EXACTLY why its difficult to fire.

You said its just a point and click weapon... but for consecutive use, its much harder.

You'll need to aim it, fire, brace it (presumably with the butt against your chest), crank it, then return the butt to the crook of your shoulder to re-aim all in just over 6 seconds... That sounds like a feat to me. lol

The crank mechanism is fairly simple, its getting into the muscle memory groove that allows you to invest enough time to aim propery that you need to account for. Trying to rush that complex combo would surely throw off your aim.

But still, what you say means that to use the weapon, one needs training. Ok, but that's the same as with every other martial weapon.

A normal X-bow is a simple weapon, maybe due to its ease-of-use. If the repeating version needs training, then it should go into the Martial category, not the Exotic. I see the Exotic category as those rare, culture-specific, odd looking weapons that require special training besides what would be needed with a normal version of the weapon.

Example: the Elven swords in the Races of the Wild. They have the same size as normal swords (short, long and great swords) but are used in a slightly different way, that allows the user to wield them with more dexterous stances (hence, being eligible for W.Finesse feats). I totally understand the idea of these weapons being 'Exotic' since they're in exclusive possession of the Elven population and the Masters of its fighting stance are all Elven, thus, you would have to meet them in order to work with the sword AND you'd have to re-learn how to use a sword properly, if you already knew how to use a standard sword.

I don't see this kind of justification in the Repeating X-bow.

Hope this clarifies the subject a bit more.

It is a more eloquent point than any I have made, and raises a point about training that I didn't think about.