Yes, the causes and effects are understood by the historians of today. They spend their entire lives studying history and the causes of certain events, and they know what they are doing. We have some really great professionals in the field, that can accurately predict what happened centuries ago around the world.
It has been 70 years since World War II. The causes are pretty clear--the lingering effects of World War I and the Great Depression caused World War II. The effects of the war are still being felt. Baby boomers are growing older and nearing retirement. In 20 years, that generation will cease to exist. Hate speech is banned in many European countries because of Adolf Hitler. Those are just two of the many lasting effects the war had on humanity.
I would have to assume the causes and effects of World War II are fully understood by 21st century historians. Your real historian scholars hold Ph.D's in their specified field and have spent many years studying their field. I would have to assume in that amount of time they developed enough knowledge about the second World War to understand it fairly thoroughly.
The causes and effects of World War II are not fully understood by 21st century historians. One detail that is often forgotten and not disseminated enough is the fact that more non-Jews than Jews died in the death camps of the Nazis. The stories of people like the Latvians, Slavs, Greeks, Armenians, and more, who were killed in the death camps are mostly forgotten.
If historians truly understood all the causes and effects of the second World War, they would have or should have spread this message so that the hate talk and the hate crimes that still happen against many different populations would not be seen as normal in certain circles. This is a complex war that had many causes, but certainly its results were horrible.