I hope this is the last mega-crossover they do, because it's hard to keep a show's storylines strong on their own terms when you have to devote so much time and attention to setting up the big crossover event. (Sort of like how Star Trek: TNG's final season and Generations were both weaker than they could've been because the producers were trying to juggle them both at once.) The Bloodwork thing just felt like marking time until Crisis happened.
The climactic threat was pretty silly. If that tiny drop of Bloodwork's goo were hit by the super-high-energy particle stream of the accelerator, it wouldn't have been dispersed across the city, it would've just been vaporized. Although even as I wrote that, I remembered that the goo was dark matter-infused, so I guess the idea would be that it would be a dark matter reaction similar to the initial explosion that created metahumans. Still, that was an important thing to say, and they never mentioned it. As described, it just didn't seem to make sense. And I don't see how adding a bit of ultraviolet to the accelerator's already intense energies would've made any real difference, let alone miraculously cured the whole city.
The scene with Cecile and Allegra getting out of the building didn't work very well either, since Cecile was supposed to be using her powers to track the zombies, but she never seemed to spot them until just half a second before she would've seen or heard them anyway, so what was the point? Also, if she was responding to fear, why did that let her sense the zombies themselves? They were beyond fear by that point. She would've been sensing the fear of their victims. So that didn't really make sense.
Also, even given that the Crisis begins on a certain day, what are the odds that it just happens to begin at the stroke of midnight at the start of that day in Central City's time zone? I mean, come on.
Anyway, now that we get the context for that Nash/Pariah preview scene, it looks even more certain that the guy behind the wall is the Anti-Monitor, and it looks like he's letting Nash believe he saved Nash's life when it was actually Allegra's UV wave.