Sync engines are fast to a point but if you start working with large enough datasets and/or care about security you ultimately end up with something closer to streaming immediate mode HTML. Of course that means sacrificing local first.
This also makes them much more vulnerable to a data leak/breach if their device gets compromised or stolen as the data is all on their device.
The client having access to only what it needs in terms of data and making that as ephemeral as possible is a big part of defence in depth.