Graduating from UNET NetworkManager

Hey guys, I'm fairly new to networking in general and am just starting to get to grips with UNET. I've finished the "Setting up a Multiplayer Project from Scratch" tutorial found [here][1] and have used the stuff I learned to make a little game that I'm happy with. The impression I'm getting from people talking about the Network Manager is that it is useful for getting used to UNET and for very simple games of a particular type, but once you want to write something serious, NetworkManager/NetworkManagerGUI is not very useful and you need to write your own stuff. I'm wondering how to get started with this? What do I need to research and learn about to take the next step and write my own networking stuff from scratch? The project I'm working on requires clients to stay in sync, and it needs to transmit data between clients but the clients don't really have "player prefabs" and I don't need spawn points as such. The NetworkManager doesn't really fit very well with my games purpose. It seems that it would make a lot of sense to write something custom rather than trying to bodge things together using the network manager. Any help/suggestions would be appreciated. Thanks. :) [1]: https://docs.unity3d.com/Manual/UNetSetup.html