I think the problem with landscape timbers is that the CCA treatment they use to get was discontinued in 2002 or 2003. Since timbers are used around the home the epa eliminated the use of CCA (the A is for arsenic) for stuff normally used there like decking boards. Some people believe the new treatments are not as affective as the old way. You can still use CCA on fence post designed for the farm.
I also have had problems with timbers warping after a while and not looking so hot when i drive by. I think timbers would still be ok for brace posts that do not touch the ground. I also have heard that timbers are often treated before they are completely dry and make not "take" the treatment completely.