For a given price range, Xiaomi hardware is probably the best in the world. And MIUI is built to maximise battery life and do more with less RAM and CPU. But this comes at the price of MIUI being very brittle and unable to handle modifications.

On the other hand, Xiaomi has good support for custom ROMs. So you can buy their phones, and immediately switch to a ROM of your choice (at the cost of losing MIUI’s efficiency).