In my experience, the "outbox" approach means a lot more manual work for developers. It requires developers to create a message for every type of change they want to sync to the client, and then also interpret that on the client. ElectricSQL's Shapes does a lot more work to keep the shape in sync between the client and the server, reducing the need for the developer to do that work.

You're right that "single-table sync" does have its limitations. At PowerSync we effectively support one level of "joins", and even then it's often not enough for more complex schemas. An older version of ElectricSQL did also actually have multi-table shape sync support, but I believe doing that at scale proved to be difficult.

One solution to this is often denormalizing data - either adding more denormalized columns in the existing table, or creating new tables dedicated to sync data. Conceptually, keeping these tables up to date is not that different from writing updates to an outbox table.

I'm also interested in seeing what Zero comes up with in the space. They seem to have solved doing multi-table query sync, but it remains to be seen how well that works in practice.