One of the main things people use us for is ease of testing writes on a per dev/agent basis which would be difficult on a read replica!

On the real world data impact I absolutely agree. We added something called "branch hooks" which essentially let you define SQL to run against the branch before it's returned

This lets you essentially anonymize and modify the branch to scrub unintended external side effects.

It's something that we're still working on though and trying to design the right abstractions around because we want to get that part right.