The Blow back option is one of those, IMHO one of those `` OOOO, Look what my gun can do´´ type of optical tunings. Yep, it is niffty, and gives a nice `` Real Steel ´´ appearance when shooting, But.....
Other downside to go along with Victorface´s post,
Not as easy to install as described,
Makes the gun quite a bit louder than normal,
Added strain and drain on the Battery,
Increases the Stress on the GB,
Increases the Stress and work load on the Body,
and probably the Biggest downside of them all, It´s just one more thing that can break, cause you undue aggravation, costs more money, and could shorten your playing time considerably.