No beta is stable or free of bugs, otherwise it wouldn't be called beta in the first place. The beta is ONLY for the Mi Box S, do not attempt to install it in the original Mi Box. Android 9 Pie brings:

1. Amazon Prime Video as a system app.
2. Dolby Digital Plus to Dolby Digital conversion for older home theaters.
3. Native support for webcams.
4. PS4 and Xbox controllers support.
5. Ability to write to USB drives.
6. Other Android 9 overall enhancements.

Even the "stable" release will have bugs.

Theres a bug if you are using Bluetooth mouse you cannot use onscreen keyboard at all while it's plugged in (I'm using a wireless dongle ) . Hulu doesn't play right, it will play for a second then the video freezes but audio continues so makes hulu completely unwatchable. . Other than those two very annoying issues the device runs a lot better than oreo did.

I find it way better than Oreo. It has a few bugs to smooth out, but it's way better ANYWAY.

I watch a lot of YouTube and sometimes a video loads up with its aspect ratio all crazy (sometimes squished vertically or horizontally) but re-starting the video usually solves the problem.

Other than that amazing beta for me.

Why cant install this to mi box 3? When they're basically same

I have a different version of box with Android 9.. but it's full of bugs and not all my apps work with 9.

No refresh rate option as far as i can see. (well stock 50/60 fps)