The first advantage for the car maker is that it requires fewer wires. If you have the right equipment its also relatively easy to troubleshoot because you can look at the datastream and see what is happening. You can also test the car's computer by injecting a simulated data set (like from a temp sensor) and verifying that everything responds correctly.
The downside for shade tree mechanics is that it requires relatively expensive equipment to perform extensive troubleshooting. Moderately priced scanners can read the codes and the datastream to provide real-time monitoring, and that can be used by the home mechanic. However, once you are able to see the problem there is not much you can do except check wire junctions and replace components.
Example: When the temp gauge started to malfunction in my 2005 Mustang, I had to troubleshoot. With a traditional system it would have been a simple wire path that ran from the sending unit up to the gauge in the dash. I could have simulated hot and cold by first grounding and then removing the sending unit wire to check the gauge. I could have checked the sending unit by measuring the resistance as the engine heated. I could have checked the wiring with a simple VOM.
However, my Mustang has CAN bus. It does not have a temp sensor in the coolant, since the computer calculates engine temp based on a cylinder head temp sender. Once the computer calculates the temp, it sends input to the gauges and makes it display a temperature. I did not have the Ford test equipment, so it took quite a while to determine that it was the gauge that was malfunctioning, not the sending unit or the wiring.