I couldn't tell you anything about their builders
That is a point that has been overlooked. As I understand it, Sitting Fox is merely a sales broker for several builders. When you buy a SF gun or kit you don't know anything about it. It is like asking "is a used car any good?".
As for DOM barrels, there was a great deal of discussion about them some time ago here. Methinks some searching will bring up much info. As I recall, the final conclusion was that DOM is suitable for our uses and has a pressure fail rate much higher than is obtained with bp.