I love the DC animated movies for the most part. This one had a very appealing artstyle (my only complain is Wonder Woman's atrocious hairstyle).
The story is okay but I do have a few problems with it. This movie has no continuity with any other DC animation and sometimes I struggle to understand who knows who and where in the timeline everything fits. As it happens, this is another "what if" scenario, where Flash meets Superman for the first time.
I know it's hard to keep track of everything since the 90s and have a perfect timeline, and nobody is obligated to predict every plothole. But its bugs me when my mind has to do mental gymnastics so that I can fit this movie in the DC multiverse. And yes, I have seen every DC animation and movie.
Another problem I have with the story is that we get to meet the Justice Society, in the 1940s. If this is your first movie, I doubt you would know who any of these heroes are (except maybe Wonder Woman). They have no time to be developed as characters and you have to guess their superpowers. Even if you know who these characters are, there are conflicts with their appearances in other media, which makes these characters new, even if you know them! Hourman for example, was is the StarGirl TV series, but there he appeared to be very different. And Black Canary had almost no screen time or character development. So it's hard to get to know her, care for her, or understand who she is even if you knew her from other animations.
The fight had almost nothing to do with the Nazis, but rather a war between DC brainwashed heroes. This is overdone and boring at this point. And Superman comes in the end to save everyone? That's lazy writing.
I wonder why they bother to introduce magic Nazi items at the start, only to never mention them again. Felt like a lost opportunity, filled with things we have seen before. A recycled story.
There were things I liked. Time-traveling is always interesting (even though overdone with Flash), Barry and Garrick sharing secrets was cool (but how come Garrick knows how to phase but not create cyclones, is beyond me) and the not-quite-time-travel plot twist was an interesting touch.
All in all I don't care about the 40s but I do like good stories. This didnt felt like War War 2, but more like DCs civil war in the 40s.