The first thing I should quantify is that I am on the other side of the planet and have never seen either the CORE or Acela builds, other than their photos. But do I have an opinion... absolutely!
I own an FG84 (FG140 equivalent) that had a coil conversion done by ATW.
The conversion was extensive, replacing all of the OEM suspension, and the ride is significantly better than any other Canter I have driven, but is it perfect... No, it is not.
Companies like ATW, EC, Acela and the like will have obviously done a significant amount of R&D in order to deliver a highly modified vehicle, but compared to a vehicle manufacturer, that R&D time/money would be insignificant.
Determining how individual components work together is a business in itself. To think that any small company could get this 100% right is fanciful, especially with the minimal quantities involved.
And then there is the after sales support... My personal experience with my own truck has been pretty good in that department, which is lucky, because I have definitely experienced issues with the modifications that were done to my truck.
Many of the modifications done to my truck were also developed in house by ATW, which means that if something goes wrong in that area there are no "off the shelf" solutions available. This could result in a hefty repair bill, if you need to pay someone else to fix a custom component.
If buying one of these seriously modified trucks you need to accept the negatives, as well as the positives of going down that path.
Do I regret buying my truck? No, but there have been issues that have seriously raised my stress levels at times.
Just sayin'...