Reliability, feed, speed are based on characteristics such as inherent design, manufacturing quality, ammunition, and maintenance. They are not necessarily dependent on whether firearm is a blow back or gas operated system.
Blow back is not necessarily designed for full auto only. There several machine guns, like the M2, M1919, MG42, MG3, that use recoil operated mechanisms, which are technically
not blow back operated. Heckler and Koch has produced both full and semi-auto guns that use delayed blow back (HK91, G3, MG21, MP5).
In general, gas operated weapons (AR15,AK,FAL,M14) that use center fire cartridges have the following advantages:
- less weight weight compared to delayed blow back
- heat and power fouling are isolated from breech (it's debatable with AR15/M16 )
- gas system can be adjusted for different loads
Delayed blow-back weapons (FAMAS, G3/HK91, Sig PE57/510) usually
- weigh more compared to gas-operated guns
- require more intricate machining operations to manufacture
- may have a small edge in accuracy and reliability compared to gas operation
There are more gas operated weapons in service because they are lighter and easier to manufacture compared delayed weapons that use delayed blow back.