Thank you for your answer, I believe I understood the fact...
If normally host and peer have the object synchronized everything should look fine... The fact that the container is not supported must mean that if the sprite peer was synchronized the text object was not...
If we put the text object on the host to show the sprite peer's life, this has to be shown on the peer's screen too... Because being in the container is no use...
Well, I think this sentence from the manual can be understood as:
Container objects of a synced object IS NOT synced. Therefore, since the peer updates the data of the text object, it did not need to be synchronized.