Smith's techinique is unorthodox, but not flawed. There is a difference. Smith has all the bases covered. He is in the right position to play all the shots, it just looks odd.
Fawad's technique is unorthodox (which is fine) but it is also flawed. He is restricted due to that single trigger movement of closing off his stance at the last moment, which causes him to lose balance and therefore would struggle to play pacers on the legside and would be very susceptible to LBW. Also the closing stnace means he would very foten be out edging to the keeper down the leg side. 
Closing his stance at the last minutes also means his weight is too much on the front foot so not only is it difficult to go back, but it is also difficult to move your foot quickly to the ball when it is wide or on the leg stump. 
The reason Fawad is doing well at domestic is because he has good hands, but at a higher level that is not enough to save you when you have such a major issue with your technique. Australia found him out in 2014 and we saw the result of that. Bangladesh then knew about his weakness and exploitet it too. 
Fawad could have been a brilliant player had he fixed this issue, because he has very good hand eye coordination, which has resulted in him being a top domestic performer.
Chanderpaul also had the same style as Fawad, but his technique was not flawed, there is a big difference between unorthodox and bad technique.