I actually really enjoyed this movie. *some spoilers*
I dont mind not seeing the monster. The idea was clearly stated that it was different for everyone, so make your own. Also you sort of do get to see it through the drawings of one lunatic in the movie. Otherwise if you have some imagination you'll probably enjoy it. Although I cant say I wasnt desperately waiting to see the monster the whole time haha.
I dont believe it is a plot hole that some people are affected differently as that's just life. Some people drink and get angry, some get happy, and some tell everyone they love them. Other post-apocalyptic movies have people who are immune. The movie even explicitly stated WHO is affected differently multiple times.
There are a few things I believe could have been better explained, like where are all the animals and are birds affected? At the same time the movie is 2 hrs already and you cant fit everything in that you want. I think this may have been better as a mini series, but I'm sure even then people would find something to complain about.
It's not the best movie ever but it's much better than many of these reviews give it credit for. I really enjoyed this movie