The article was paywalled, but I'd imagine it's two main difficulties. One is fairly normal for early-production, but the other is much more fundamental to the design:

* The battery - Tesla is using some new battery technology on this model, which they haven't done at scale before. This will come together with time, as it did on the S, 3, Y, etc.

* The body - Unlike practically every car on the road, the CT is made out of stainless steel. Instead of stamping typically used on body panels, they have to be bent on very precise folding machines. The higher strength causes a lot more wear on the dies used for stamping. The only other production car to be made with stainless was the Delorean, which was also a mass-production nightmare.

https://wisconsinmetaltech.com/why-dont-we-see-more-stainles...