Well, you cannot print steel springs, a steel bolt or bolt carrier, or a steel barrel. And the plastic firing pin just does not do very well.
As said- they are printing the parts of an AR type rifle that are not subject to high pressures. Shoot- you could make a lower out of a block of seasoned hardwood if you were so inclined. HAVE been them made out of heavy plastics. But that is the LOWER, not the upper.