You're right, it certainly could make debugging harder for large apps, but the point is to stop building large apps. Since every app made using (fab) is itself a (fab) app, debugging/testing should take place at the component levels first.
This presentation was awesome. In particular, the hydration/dehydration explanation at 51:35 was deliciously evil: replacing ones and zeroes with tabs and spaces to not just obfuscate javascript, but actually hide it from human view altogether.