Take for example the common turning circle, if you try to break it up into 4 blocks with chain signals. it works fine until a train decides to reroute in the middle of the turning circle and decides to leave through an exit the train has already passed, the result being it tries to go all the way around the circle, but it can't because its picking an exit that intersects with itself, blocking its own exit because its in the middle of a set of chain signals.
Example image: of issue with path hilighted.
