First off, you're comparing Quicksilver and Scarlet Widow? to Flash.
Second, I never said they were properly introduced in Ultron. They're barely characters but ultimately it doesn't even matter because they're not remotely as important for the Avengers as Flash is for the Justice League.
Also Ultron of all things was still a better movie than anything Snyder and I'm 100% sure it'll end up being better than JL unless Whedon makes a miracle, so it has that.
I'm saying your criticism is stupid given Flash and Aquaman can both be introduced in a sentence each.
Flash - Guy hit by lightning who runs really fast.
Aquaman - King of Atlantis.
That's all you need. And my AOU comments were a joke, pointing out that Quicksilver and Scarlet Witch have ludicrous comic book backstories and the film simplified them down to "he's fast, she's crazy."
Not to mention how Flash literally has a show running right now watched by millions each week. People know how he gets his powers.