We must remember that the DCEU is a universe that’s already there… by that I mean that we’ve entered it and everything is already in motion, kinda like real life. When you enter college, you meet new friends who’ve already grown enough to know what they want and you meet others who are now starting to discover who they really are. You don’t know their whole story, you never will, and you don’t need to. You’ll see them in action, you’ll see them develop more into what they want or what their situations mold them to be, you’ll be with a friend or multiple at a time, but you don’t need to know everything about them. In Man of Steel, the viewer joins the DCEU with Superman, who is also just entering it. In BvS and Suicide Squad, both Superman and the viewer realize that the world has been going/moving/turning long before they join, so characters are established, relationships are formed/destroyed/mended, yet the new additions don’t break the coherence of the world, rather elaborate on it by showing that there’s a much bigger picture but we can’t just see it, just like a freshman entering college.
Any critic panning the DCEU for not introducing characters beforehand, or for making “incoherent” stories that follow “too many characters” at once, clearly doesn’t know how this world works either.