I find it believable, but unimportant. If you have your bags loaded... and I mean full&heavy, they are more unstable (until you adjust to it). I doubt that a wider rack makes a bike anything beyond marginally more unstable. I think it's more a matter of looks and need. As I mentioned above, I have the Bontrager Back-rack and it's an ideal width for touring with plenty of room for a tent and sleeping bag and some other odds&ends; it also has plenty of room for tires/fenders. It's wider than I need for a general commute with one bag, so that's a tradeoff. Likewise, wide vs narrow isn't the only factor to consider, how high the bags sit will also influence stability (lower is better).

So, I wouldn't say that wider necessarily causes instability or that it would be noticeable. I think it's case by case and depends on the bike, the weight of the load, the height of the bag and tradeoffs such as tire size and other potential clearance issues.

Another rack worth considering is the PDW Payload which in my opinion is the best looking rack out there after Tubus.