My Xiaomi MI3 (international version) is almost two years old and now got automatically upgraded to Android 6.0.
(No doze support and it revoked my root access but still nice!).
(No doze support and it revoked my root access but still nice!).
My guess is that Xiaomi already has a similar feature (perhaps less sophisticated) called "Autostart". Most app aren't allowed by default to wake up in the background and kill your battery. Only some (e.g., whatsapp) are whitelisted and, of course, you may whitelist additional apps yourself.
Overall, I'm happy with the battery usage of my Mi3. I prefer the Xiaomi Apple-like UI over Google's standard -- but that may be just a habit...