Agree with your sentiment of Dunkirk. It wasn't bad but I expected a bit more.
I cant put my finger on why but something about the trailer gave me the impression that would be the case for me too. As noted by @Batman, being a Nolan flick I know the production value and cinematography would be top notch.
I watched Darkest Hour this past weekend and was hugely impressed by Gary Oldman's performance - as well as the supporting cast.
From the moment I saw Oldman in the Churchill role I was excited about this one.
To be honest there is so much Churchill material available that I'm shocked there aren't a lot more feature films around his life being put out by A list actors and production teams.