Well, for one thing, they seem to be pretty hearty and resilient. For example, we planted a few awhile back - some that we found discarded on the ground. We don't know how long they had been lying exposed but they didn't look very good and we didn't have a great deal of confidence that they would survive. But we gave them a chance and were we ever surprised!
Here's the prickly pear - the greener pads are all new growth:
Not sure what this guy is but he's firmed back up (regained turgor) and feels well-rooted:
This Ocotillo (shrub actually) appears dead and yet we are confident that, given a bit more time, it will leaf out and thrive:
Here's an agave (Agave lechugilla) that has greened back up considerably:
Our only for-sure casualty is this Cholla cactus:
The main thing with these sorts of plants is that they really are pretty hearty and they actually do better if you let them dry out a bit when transplanting. The other main thing that we've learned is to not be afraid to try - there's not much to lose especially when working with discarded and forgotten plants. If you are in xeriscape country, go for it!!