First of all, the concept is good and had so much potential.
Cons-
1. The relationship between william and elishia could've been a bit more elaborate, like in what way were they connected
2. What was the purpose of the whistle thing that william had apart from creating that pattern on grass, does it make dead ppl rise? If so then at the last season before sacrificing themselves in the fire why did he blow it?
3. In attempt to bring elishia to life, william blew the whistle and 2 ppl rose chi and the 15 yr old cristian girl, then next day he blew the whistle again and 2 more people rose that were never showed to us in the series, so how did the disaster at the end of the series stop if they were alive?
4. The series is extremely monotonous with de*d people just running away here and there. The series could've been cut really short and makers should have focused more on ending or the science rather than their past lives. I had to skip forward so many unnecessary monotonous parts of the series that didn't add any substance to the story. If they worked on these points series could've been a bit better