There are potential roadblocks at every step of this process. Few people understand how to deal with embedded systems failures. It's difficult to determine which embedded systems need to be checked. It's difficult to assess the internal logic of the embedded system. The scale of the problem is so great there are not enough trained technicians capable of working on them.
Testing can damage the system and cause a malfunction.
Because of the multitude of models and versions of embedded systems, it is not possible to extrapolate from one system to another based on the testing of one. Identical chips may act differently in different systems. In cases where a replacement chip is required, it may not be possible to identify the vendor, or the vendor may be out of business. It may not be feasible to manufacture a replacement chip. Malfunction of an embedded system may trigger other failures, and the source of those failures may not necessarily be detectable.