This weekend I used the Mi Box (now running Marshmallow) extensively. My Netflix is stable, but started getting random popups with "VUDU has stopped working". At the same time I also noticed YouTube randomly freezes. The app stops working completely, it just freezes. I had to reboot to get it to work again, after which it just continued to freeze again. I uninstalled VUDU and all the other apps that come with the Mi Box (that typically dont work in SA). YouTube seems fine again.
I also discovered yesterday that HDMI CEC does not work in Marshmallow, something I really wanted. This worked great in Oreo, but of course I was advised not to upgrade to Oreo and hence HDMI CEC wont work.
I'm really getting to the point where I dont think I'll recommend this box to anyone. It just doesn't feel like a stable box. Maybe Oreo fixes some of this, but I've also read many issues with Oreo and was also advised to not upgrade to Orea. So it's a catch 22, run older Android and deal with issues or upgrade and risk other things breaking without a path for downgrade again.