I built up a flattop M4gery J&T kit (same company as Doublestar) on a DPMS lower. There was one part missing from the kit, the company apologized and replaced the part with no hassle at all.
Three years and at least a couple thousand rounds and so far no problems. Good company to deal with. If you call them, there’s a good chance you’ll get the owner on the phone.
I have a Doublestar lower. Color match and finish quality seem as good as the DPMS and a Rock River. Still need to decide what it should look like when it grows up :)