For the sake of cheapness there is nothing wrong with the S912's other then the typical lack of support for some of the boxes hardware features when running under Linux (LE). It just means more things need to be done in software. I have good 4k tv's in the house and typically stream most incoming stuff in 1080 without any issues, as well i have a huge storage network which everyone in the house streams local content from and even some of the 4k stuff still works good enough for our use. I would suggest tho whatever you buy try and get at least 2g of ddr.
Some of the newer x3 boxes like the A95x3-f3 because of the newer mt7668 chipset have no wifi-bluetooth in LE or any of the other current forks at this time. The other ones you mention you might need to do a bit of checking to see if they are in the same boat as i seem to remember some of the Tanix boxes were using a old no longer support wifi chipset. I quess you would need to figure which exact box your looking at and then just check and see if theres anyone bulding for them and what issues if any people were having with them.