So, now that the entire story is in retrospect, a couple of thoughts. Spoiler alert...
While enjoyable, I don't think that it held up to how well the first two episodes [which were shown back-to-back on the same day] set the story up. It started out VERY compelling, but kind of meandered.
Lots of untidy loose ends. Was the documentary / interview purely just a plot device? Seemed like there was a lot of potential there that just got dropped. Wouldn't it have been much more impactful if the Sara Gadon character had been Julie Purcell... or maybe even the daughter of that cop they killed -- trying to solve THAT crime? Of if she'd come at Hays with evidence of some atrocity they'd committed, versus unsubstantiated theories? And what was the point of revealing the son's affair with Elisa? This whole thing seemed very important, and then just kind of faded out into story irrelevancy.
I also felt like we didn't get enough of what happened after the rift in the 90s between Hays and his wife. That didn't get adequately resolved, IMO. Did she leave him? Did that take its toll on their kids? How did she die? And as Steve pointed out above -- they seemed ostentatious about the daughter being estranged [or possibly even dead] given that he was hallucinating about her as a kid. So to have that be fairly ho-hum in the final episode didn't make a ton of sense.
I liked the inclusion of the Hoyt's, but it was very clunky and easy to spot how it played out. I find it difficult to believe that detectives / law enforcement wouldn't have put the pieces of that puzzle together. I also didn't like that the one-eyed man explained everything via a monologue -- that is just weak sauce writing, IMO -- I'll admit, that's a personal pet peeve, and others may not have felt the same way.
I also thought that the kid who apparently ended up with Julie at the end was something they beat us over the head with -- with the over-the-top wave in the first episode, then the ostentatious landscaping van sign in episode 7, and then the chance encounter with Mike and "Lucy" in episode 8. Very ham handed.
On a more positive note, I did enjoy the series. Letting the story unfold across three different points in time, with one of them being from the perspective of an old man with dementia trying to mentally connect the dots, was an interesting technique that blended together well and was intricately designed. I thought it was fitting somehow that old Hays solves the crime, but loses his memory right as he's about to go talk to Julie -- even though we as viewers got to "experience" what the resolution to her story was.
And in the end, I think it was well acted, with characters who were so interesting that they quickly took precedence over the story backdrop of the kidnapping itself. But I feel like the resolution of the kidnapping case was kind of contrived, almost as if it became unimportant compared to the character development, and thus was rushed into being wrapped up with a neat bow at the end.
Overall, it was a good watch. Started out great, loved the characters, finished good but not great. Better than most stuff on TV, but probably not as good as what people suggest season 1 was.