There's no way I'm going to use treated SYP or railroad ties around my vegetable garden. I have used northern white cedar but was disappointed that it lasted only 15 or 16 years. As a furniture maker who has used some locust, I would say the market for furniture wood is very limited. It is ugly to work with. Maybe it would be appropriate for garden furniture, though.
Black locust is very rot resistant.
If moisture content is carefully controlled and the drying process slowed down, cracking is minimized. Black locust has great structural strength and longevity. A lot of these trees were planted at the time of the California Gold Rush and used in mines to shore up the shafts. I have seen the lumber used all over the gold country for support timbers in old barns and in homes and also for flooring, stairs, mantles, etc. I have a lot of black locust on my farm, which was planted by the original settlers.