Whedon shouldn't worry. DC are as committed to the DCMU as Italy was to the Second World War. Yeah, you did well with a couple previous outings, but you're doing too much too fast, bitten off more than you can chew. Bat's V Sups has to introduce what, 4 new heroes and (Maybe) 2 Villains.
Granted, Batman is Batman, but still, you have to introduce him in this new world you have to build. Then Wonder Woman, who's origin is baffling and weird, given how often they change, Lex Luthor is fairly straight forward, money and power corrupts, but then Aquaman and Cyborg, who have been oddly absent from promotional, which hints to some shame that they're involved. That's another issue, shame. If no one making these seems proud to have Aquaman, god of the ocean, or Cyborg, badass super smart cyborg (Robocop but with emotions), in their production, that speaks volumes for the rest of the universe.
How do you introduce the fantastical elements of the JLA like Zatanna or the Martian Manhunter? Will they be ignored in fear of being laughed out the cinema or will they make them Grimdark and 'Realistic'?
And, on my own personal note, why the hell should I care for a movie franchise that so far has shown me that all it want's is Grimdark serious? And has shown me that it shall keep that? It's not like Flash or Arrow are being part of it, a misstep if any.
At least Marvel took there time before the Money shot that was Avengers. DC seem to just want to shoot their load of the JLA as soon as possible so it doesn't get embarrassed by it.