Yasir has been a really poor bowler outside asia but you have to cut the guy some slack instead of bashing him. He is not better than aswin or lyon but more valuable than any fast bowler we have produced in the last 10 years barring maybe shaheen. It also some of it comes down to support from the pacers. Yasir has to bowl the majority of overs and he get 1/3 rd of the wickets. Ashwin gets 29 % of the teams wickets and lyon 22%. This is also a reason why i rate murlitharan greater than warne as he took nearly 40% of his teams wickets and carried a weak bowling attack.
its not his wicket taking ability its his run rate he currently goes at but i dont blame him tottaly i blame the management for playing wrong spinners in conditions