Technically speaking a conforming agent can ignore every single sendBeacon call by always returning false. And that's a point: beacons are opportunistic asynchronous requests. You can't do anything when they error anyway, so they should be regarded as hints that agents may or may not honor.

Okay so it's basically just a baked in implementation of what people did already but instead of using onUnload they have more visibility into the client context. Seems dirty in a way, the agent could basically internally call the same thing and send any data to their own server on every page?

For example, firefox, If they wanted, could record all your browsing history without notifying you? For any bad actor this seems like an amazing attack point.