Tesla opts to do repairs centrally. So when a unit requires service - by analogy, anything from timing chain adjustment to head gasket to valve job - they have it sent to Fremont and replace with a refurbished unit. This promotes quality by giving R&D direct insight into service requirements so they can continuously improve.
In theory, this sounds fine except when you get multiple failures and refurbished DU that were defective from the start and had to be redone, this tells me that they either don't know what's causing the issue or they don't know how to solve it.
Some are here stewing, interpreting each drivetrain repair as major issue. They're not.
Leaky pano roofs, tail light condensation, misaligned trim, etc. Not a major issue. Any thing that prevents the car from working is a major issue. Also the guy that had his warranty refused was qouted 15k to replace his DU. Sounds pretty major to me.