They're certainly more complex as a result of higher quality demands. A car can be tested under real-world conditions and fixed if it shows any manufacturing defects. Any mistakes with a rocket are very expensive.
Testing is also very different. For a car, you can build a batch and test it extensively under all conditions, fixing errors as they come up. If you develop an assistance system, you can test this extensively before releasing it. Rockets are so expensive that SpaceX had only a few trials for their landing system. You need to make sure that both hardware and software work perfectly without ever testing them in production.
It's worth noting that one big advantage SpaceX has over other rocket makers is that SpaceX gets to inspect landed first stages. That makes their rocket development closer to airplane development than everyone else's rocket development, and no, it doesn't have to be perfect before it is tested. It just has to be good enough to land.
Testing is also very different. For a car, you can build a batch and test it extensively under all conditions, fixing errors as they come up. If you develop an assistance system, you can test this extensively before releasing it. Rockets are so expensive that SpaceX had only a few trials for their landing system. You need to make sure that both hardware and software work perfectly without ever testing them in production.