Floating definitions.....
Is it possible to build a combat weapon system at home that could be trusted in austere conditions and hold up? Yes
How do you know? Has it been made in sufficent numbers (exact parts, assembly, same builder, ect) that it can be tested in numbers with reliable enough data to be trusted to?
The truth is, you have zero fucking idea. The time to find out your little dream mixmaster of parts will shit the bed, is not in the middle of a gunfight or on a $10K hunting trip.
You trust what is proven, and there are VERY few companies that are proven. HK, LMT, KAC, FN, COLT, BCM are on that short list. They all are Proven, in numbers, by the most demanding military units in the world. When you buy one, You are buying that engineering, experince, testing and proof, that they WILL WORK (or atleast should). Google machine some of the testing protocals used durring military weapon testing and it will blow you away with the type of shit they put them through. Testing/QC is a large reason why all those above named companies are at the prices they command.
So saying shit like POF or Ruger or LWRC or any of those commerical companies pumping out shit as fast as they can are on the same level as the above mentioned compnaies is fucking ludacris.