I have conducted many PQR test using FCAW E71T-1 on A588 and A572 steels, per the AWS D1.5 fracture control plan. Test specimens were per AWS Fuigure 5.1 (Reduced section tensile, all weld metal tensile, side bends, CVN, macroetch, and chemical analyses.) Almost every test has passed; those that didn't were the result of something the welder made a mistake on. We have used FCAW for everything from API 650 tanks, to bridge members, to structural steel for many years.
Based on that, I have to say that FCAW is certainly capable of producing good welds. You do have to be careful in that a lot of poor welding can be done quickly with just a pull of the trigger. And heat input is very important.
But I suggest that you conduct your own PQR testing so you can check the mechanical properties yourself. I think you might find your problems could be solved with additional welder training and testing, and quality inspection before, during, and after welding.
Chet Guilford