I mean, the downside is that it is a potential stumbling block for their already successful and well respected Batman franchise. A franchise which, frankly, has a far greater chance of being at least a mildly successful shared universe than the DCU does. It creates an inherent potential messiness and also opens up conflict between the two parties over who gets to do what with which characters. I know they're talking a big game that everything is fine, maybe it is, but it is clearly not an ideal scenario. That's part of why I find this movies existence is so absurdly bankrupt and lame: they're already winning with Batman. They've laid the foundations for it, it has enormous room to grow but in the current cinematic climate that isn't enough - there has to be another one so he can run show up in the Justice League, there has to be another one to tick a box on a checklist to accomplish something they've been trying and failing to accomplish since Green Lantern.