Third Party Mirror : SyncLists and SyncVars on Player objects

I managed to get SyncLists and SyncVars to work on a regular game object, but it seems like nothing I do can make these work on Player objects (cloned from a prefab). I can see on the server the Player clone has been created and data populated. The Player object gets spawned as expected on the client, but SyncLists are empty and SyncVars are uninitialised. Any clues?

 

I got a sort of resolution to this by reading this article:
https://mirror-networking.gitbook.io/docs/guides/synchronization/synclists

and noting that:
QUOTE: "SyncList must be declared readonly and initialized in the constructor."

Once I had readonly in place, it made me re-write some code and subsequently everything worked.
But I don't completely understand why what I was doing before on regular Gameobjects also worked without readonly in place. (But that is a question for another time.)