ECS world replication

It is hacky and and we do hope to change to native ECS data serialization similar to what you describe at some point.

Early on we (the FPS Sample team) settled on a very simple and manual approach to serialization as we wanted to focus on other networking problems (e.g compression algorithms) and general gamecode. Our solution have now mutated into something that also supports pure ECS entity replication by wrapper classes.
A lot of work is being done on ECS and it is still experimental, so I think for the time being we will keep this "whatever works" mentality for the actual serialization.
I also think we have a more important task to solve first: how to make it transparent who data is replicated to and how it is handled on client (e.g. predicted and interpolated). It should be easy to create a new replicated object and define how different sets of data should be handled (local non-replicated, replicated to all, replicated to owner, replicated to non owners). And currently it is .. not simple.