I think the biggest problems experienced with ivory is the difference between the climate in which the knife is made vs the climate in which the owner resides.
If the knife is made in the midwest, then shipped to a very dry climate such as Las Vegas, the lower humidity will draw the moisture from the knife and cause the cracking, shrinking, etc. If the knife is made in the same climate in which it is used, there is less of a chance of that happening.
I know that Tony made me a back pocket knife several years ago that I have owned, carried and used consistently since I received it and the only thing it has done is change color (darkened) and shown a small amount of shrinkage which decreases based on the season. It has been a fine piece and a great user. Stag, wood, bone and other natural materials (except for pearl) will move as well in varying degrees.
Again, the key is really the diversity between the climate the knife is made in versus the climate that it lives in long term.