About end of last month I get this MT7688 board from taobao.
Just have a try recently, result is here:

Good Parts:
1. Temperature is low, about 35C.
2. Better memory, 64MB.

Bad Parts:
1. Wifi signal is very weak, about 5m it drops to -90db. (I think that is antenna problem, clear zone is too small)
2. The chip provider said this chip cost three times more than RT5350.
3. OpenWrt do not have a good support yet, need more contributors, but all datasheet is not open.

Unknown Parts:
1. Power consume is unknown yet, I use 9V power, the average current >0.30A, maybe the LDO power consume is too much.

If I have more time, will do more test later.