For anyone else encountering this:

This affects 2017.1.1f1
It occurs for any kind of sync list as far as I can tell (SyncListBool SyncListInt, SyncListStruct etc).
It only occurs if the class is not directly descended from UnityNetworkBehaviour.

e.g.

class CustomBehaviour : NetworkBehaviour
{
private SyncListBool syncList; //this will work
}
class HangBehaviour: CustomBehaviour
{
private SyncListBool hangList; //this line will cause the hang
}

I've worked around it locally by moving the SyncList declarations into base class and then only setting them up the classes that use them - not ideal but it works till a fix is released.