The film is underrated for sure. For a movie not meant to make headlines, it over achieves.
Realism: in movie specific situations, the viewer must be able to relate. When things happen in horror movies I can't help but say "people don't act like that, they're all dumb" this movie is exactly how people would act in this situation, which is why many people are criticizing harshly, they have been spoon fed rediculous acting and emotions in movies for so long.
Acting: at no point during the movie was there noticably bad acting, sentences and word framing were real, expressions were on point, body language spoke for itself
Premise/story: the story was a much more realistic version of any rediculous frankenstein movie or book. The people that don't like it must not understand evolution. They are not bringing back dead people, they prolonging the recovery time frame from when someone flatlines which would save millions of lives and avoid brain damage. (It goes horribly wrong obviously, and you can feel the impact and emotion from the actors)
Conclusion for viewers: do not watch this if you are not smart enough to follow a basic scientic premise; without that it would be a cheap story to the illwitted that resembles a rehash.