Archive 81 is a fine series that limits itself in many ways.
It never goes full mystery and never tries to bend your mind and leave you confused for a couple of episodes.
What it does most of the time is it builds up to something, leaves it right away, builds up 5 new things and 2-3 episodes later tries to pick up on these but gives up. Some episodes are also tonally very different, which shows in random motivation and attitude changes, like when our main lead suddenly goes Oceans 11.
Surprisingly enough, the most mysterious episodes were done by Benson&Moorhead duo, although I dislike their other films.
If you strip the series of all the basic "cult" shenanigans (overused by Kill List, Hereditary, The Empty Man), the story becomes so simplistic they sum it up in a few sentences in the last episode.
The series has a nice feel to it, the atmosphere is great, although sometimes it's grainy, sometimes not, and color correction changes, there's probably a reason for that.
Acting-wise only the supporting cast performances can be enjoyed (although sometimes completely misdirected), the main archivist lead is wooden and a complete bore, lost girl Melody is memorable, but not due to the acting, she's unique in some way, but I can not say for sure why. Sadly we didn't get enough supporting characters on screen. Even though some of them were barely believable, like the Davenport brother, who is so useless you can't buy him for a powerful company CEO.
The 90ties setting was ok, they didn't go full nostalgia, but kept some of the pop culture references.
If the ending wasn't such a cliched failure, I'd give Archive 81 an 8-9. It is still a good watch, just drags a lot, wasting too much time on boring leads. Additional creativity would've greatly improved this. Like seriously, if the most intriguing part in your series is the otherworld and the demon-god, at least try to make these look good.