I have an AR15 in 7.62x39. There has been good results with accuracy, with all types of ammo in this platform.
The buffer, spring, bolt, and hammer spring in some cases, need to be changed for total reliability. There has been ftf's with the hard primered ammo (wolf, etc.). There has also been some bolt breakage. But with some adjustments, the AR15 7.62x39 has merit.
Recently C-Products has come out with a reliable (30) round magazine.
There are piston kits in R&D, that will work with the 7.62x39 AR15 platform.
7.62x39 ammo is proven, still abundant, and cheaper, than