Why the low score?

I quite enjoyed the first half of the film... Really liked the father and son relationship and the struggle with reality (although I wasn't really a fan of the dream sequences). It only got silly when they introduced the gangster element and trying to sell him to the real Jamie.. They didn't really need that and I thought that turned it into a completely different film, I really felt for Frank because he was obviously scared that he was loosing his mind in the flashback sequences. It was obviously also quite inconsistent with how bad Frank was but I suppose people who really have Alzheimer's also have moments of clarity and are better at remembering things long ago.. I just wish they had kept it as a struggle for a son trying to help his dad.