I don’t think the QC on the 121’s is any worse than any other modern detailed locomotive. . I have 7 and the only problem I have had is delivery with the cab end vacuum pipe broken on 3, which obviously occurred due to the packaging.
I would say that I found product support for replacement parts for the broken parts to be nonexistent. Contact with Murphy and I received an email to say that they don’t deal directly with the purchaser, see your retailer. The retailer couldn’t get any spares either. Disappointing.
Another area that could be improved is the internal packaging. I don’t think the foam cradle idea worked particularly well on this occasion.