I didn't like them because they felt flimsy, and only had two straps on the front. The person who brought them for his eventers sent them back because they just didn't seem tough enough to trust.
I use the front one's on my horse XC and never had a problem, the bit that runs down the back of the tendons is hard so no worrys of the horse stricking into the back of their legs. There not flimsy in anyway, all new boots are lightweight but tough.