I had a Mi box for a week or so back when it was first released and ended up going with a Shield instead because of the bugs that the Mi box suffered from. But now I find myself needing another streaming box for another TV for Plex/YouTube/Netflix and am wondering if the Mi box has received any updates that have improved it's performance?
I'm not sure what issues you had previously, but I use a MiBox as a secondary, and have no problems playing the typical streaming content. Also have the ShieldTv as a primary device. I've only had one update on the MiBox in the last 4 months or so, which didn't seem to change much for me.
If you are in the US, then give it a shot from WM, and if it still doesn't meet your requirements, it's an easy return.