I think I've cracked the Wifi issue with the Mi Box. I believe the Wifi issue depends heavily on the make and model of wifi router you're using and the channel bandwidth setting you have for your wifi channel. After buying this box for myself and helping a friend buy one and set it up, I've noticed a few things.
I bought a Mi Box for myself a few months ago. I connected it to my dual-band router on the 5GHZ band. My channel bandwidth was set to 80Mhz. I was suffering from constant disconnects. I then changed the channel bandwidth to 20Mhz/40Mhz/80Mhz and the wifi has worked flawlessly ever since. This router was the Asus DSL-AC52U.
I then helped a friend of mine to set up his Mi Box. He has a DLink DIR-825. The channel bandwidth on his 5GHZ network was set to 20Mhz/40Mhz/80Mhz but he was still suffering from disconnects. I then changed it to 40Mhz, still the same story. Then I changed it all the way down to 20Mhz and it worked flawlessly since then.
So it seems that depending on the make and model of router, and what channel bandwidth you select, it can affect how well the WIFI connection on the Mi Box performs. There must be a problem with the compatibility between the Mi Box wifi radio and the wifi radio of the router. However the lower you set the channel bandwidth the more likely it is you will get the wifi working with your router.