I wonder how much of this is Boeing and how much is NASA. It's very easy to throw Boeing under the bus right now, and they might even deserve it, but at a minimum, NASA deserves blame for not doing due diligence. They're also trying to build something that we've largely forgotten how to build. It's a different NASA, and different Boeing, and design technology and manufacturing is completely different. It isn't reasonable to expect this to go smoothly.
I think Nasa has forgotten how to Nasa because of SpaceX. They still cannot adapt to how fast SpaceX moves. They are used to being able to plan everything for years as technology moved slowly. Now their plans are obsolete by launch date.
Boeing appears to have simply collapsed under its own hubris.
Boeing haven't exactly been covering themselves with glory with their non-NASA work over the last decade or two, either. But I'd still go with "both" :)
The blame should honestly go to Congress, because they keep interfering in the process to force NASA to stick with SLS. NASA has tried several times to ditch this albatross around their necks only to be scolded by influential senators determined to keep money flowing into their states despite the overall project failing wildly.
