Its not about minix being special or anything, but which box is more suitable for noob users and which box has better aftersale support. Case in point, now you having problems with your mibox 3, have you managed to find any help at all? Try searching for anything on xiaomi boxes and 90% of stuff you get are in chinese. Good luck trying to solve problems relating to mibox when there are precious few avenues where you can seek help from.
On the other hand, apart from regular firmware updates, minix has an active english forum where you can find or ask for help when you have issues, even for their older models from 2-3 years ago. And since minix is one of the more reputable brands out there, there are plenty of other sites and forums on the internet which provide discussions or resources useful for end users. But all these comes at a cost as you have to pay a higher price for getting a minix.