What are the actual root causes of Appium real-device flake?

Running Appium + XCUITest on a BrowserStack real device pool. Flake rate ~12% on iOS, ~6% on Android. Screenshots on failure look identical to passing runs. I've tried longer waits, retries, and disabling animations. Marginal improvement. For teams running Appium on real devices at scale: what are the *actual* systemic causes of flake beyond "add more waits"? Session leaks? Device state? Network variance on the cloud provider? Would love a checklist ordered by "likely to fix your problem", not a list of every possible thing.