How historically accurate is the movie Dunkirk

Dunkirk is a 2017 movie about the evacuation of allied forces from the Dunkirk region in late May to early June 1940. Operation Dynamo, as it was called, involved a mass evacuation of British, French, and other forces after allied forces were effectively trapped in northern France. The evacuation involved military and civilian sea craft and narrowly avoided a major disaster for the allies.