As is the iceberg which strikes the Titanic. These terms and principles can and sometimes do overlap. Therefore, some obstacles are also complications. Some complications are also reversals. That's not always the case.
Some events are purely obstacles.
Some are purely complications. Most reversals, however, by definition, are also complications as well a particularly distinct and major complication with an extremely sharp turn. The Death Star, too, was initially an obstacle that they sought, unsuccessfully, to avert and which then quickly develops into both a complication and a reversal. Sometimes an event will turn out to be a complication, but initially it doesn't seem to be so. Or, looked at another way, sometimes a complication will play out in two or three beats, which can be slightly or widely separated in time.