Thanks for your reply. The reason that I'd like to use these is that I want adjustable shelves and I like the fact that they disappear (aside from the extra holes, of course.) The shelves will go into a clothes closet so I'm not terribly concerned about weight. While I agree with your comment that "depth is weight capacity", I am more concerned about the stability question raised by having shelves substantially wider than the brackets.
As a result of your comments, I decided the best thing to do was test them. I build a test jig with a single shelf of my target depth (22"). The deepest brackets that I've found are just over 11", so there's an overhang of 5+" both front and back. The closet system, including shelves, will be built from 3/4" plywood. I loaded the shelf up with a good bit of weight (a large bench vise) and moved the weight to the front edge of the shelf, the weakest point. There was no failure over several days, so I conclude that for my use these things will work adequately. I started to get some cracking in the plywood shelf when I pushed down with a lot of my body weight. I think with 3/4" hardwood shelves, these things would be very strong.
Although I think that the single bracket would be sufficient for my use, I have decided to err on the side of caution and add a second set of brackets to spread the load more fully across the depth of the shelf. Now I'll have a total of about 17" of bracket for a 22" deep shelf.
One last point: the strength of the overall system will be somewhat dependent on the tolerances of the bracket holes and shelf kerfs, so I was careful to get both as snug as I could.