It's a fictional end of the world story which has real promise. Yes in parts unbelivable but that's the point of fiction. Actors are well cast but by half way through season two it just got silly for me.
I watched until the end of season 3 but in my opinion there was no need for hybrids. The animals being crazy was enough and this should have always been the driving force of the story. Also the main characters changing their minds and swapping sides got stupid too.
However I do think it's worth a watch, even if it's just first season you watch.