Maybe I missed one or two chapters here, but, why exactly SLS is messed up?
Using existing parts should mean less cost, and SLS costs much more than the Shuttle, and flies much less. The SSMEs existed, the 5-segment SRBs were first fired back in 2003, the Core should be somewhat simpler to build than the External Tank, because it doesn't have the Orbiter hanging on the side... yet, all the metrics are worst that what existed before. Plus, nothing comes back (well, Orion does, but that is the payload), while the Shuttle would only expend the External Tank.
Another thing that I think was a mistake was using SSMEs instead of a simpler engine. Not only are expensive, reusable engines ending up in the ocean, but they require several systems to support their operation, which means a Shuttle aft compartment needs to be built for each SLS... and then tested from scratch.
This is not just criticism at NASA, but also (or mostly) at Washington, which decided to build a rocket from existing parts (to keep their friends in business and voters happy) ignoring if it can perform "the mission", instead of starting with the requirements and then building something that fulfills those requirements.
A good example of how this program is managed is that some politicians mandated a Propulsion Test Article to be built
after the first Core was test fired.

