You write the Worker code which runs on our infrastructure, not in a browser. You have complete control over who can access which piece of data, as it's your code doing the accessing.
Aside, it seems like this could be really interesting combined to make an entire API with static delivery from S3 or Azure Blobs, with a backend on a cloud hosted database. With a lot of flexibility in between.
When you say overhead limit, are you talking about latency? Our goal is to keep reads on the order of 5ms in the 90th percentile.