With internal SoC's getting faster and faster todays TV's run Android TV better than ever before but I still prefer an external device to the built in OS thanks to features like frame rate switching and lossless audio support which the native OS on the TV lacks.
No clue about the Mi Box. But don't go for the Stick, that thing is afwul. Just a heads up :)
My Hisense got updated to a version of Android TV that looks close to what I get from the CCwGTV dongle that's attached to the same TV. I went with the external device since I'm banking on Google updating more frequently than Hisense but currently the TV has a pretty decent internal OS.
I have a tcl with Android. Android itself is great however the tcl just doesn't have the processing power to constantly run smooth performance. I am going to add a new Chromecast took my tcl as I believe it will perform a lot better
I have friends with a android Sony and it runs much better on that tv however
Mi-Box is where I set the bar for entry level for decent streaming boxes. Next choice would be Google Chromecast with Google TV, then a Shield TV Pro.
I have had the Nvidia Shield 2015 since 2015 and it's still my daily media device hooked up to my 65 CX.
Last year, I upgraded 2 TV's in the house to Philips 43inch 8505 TVs (They call them 'The One' or 'Performance' serise. They have FALD, 3GB of RAM and are updated constantly. I'm really surprised by them if I'm honest. They are a little slower / laggy than the Shield, but not like the old TV's that Philips used to produce that became unusable after a few days, the new ones, well this model anyway, has been great and I'd highly recommend them for sure.
I have Mi Box 3, Mi Box S (4) and Shield Pro 2015 and 2019.
Mi Box S is a very good streaming device and Shield TV is for the extra mile, Plex server, emulation, playing exotic type of files from external or network storage at high bit-rate.
If you gonna use Netflix, Hulu etc start with the $50 Mi Box S and then go for the Shield TV. Android TV is very optimized the difference between the Mi Box and Shield opening apps are 3 sec. Of course Shield has power overhead but it's overkill if you gonna using it only for Netflix.
Sony recently is the only built in one worth considering based on performance. Also go chromecast with gtv instead of mi box if not the Shield