I've been aware of the unofficial Carcosa supplement for some years, but never saw it in the wild. A couple weeks ago I picked up the version put out by and for Lamentations of the Flame Princess.
1. DriveThruRPG keeps it behind the Adults Only barrier due to it's disturbing nature. Well, if you're a 13 year old with an over-active imagination it might be disturbing. The fact that almost of of the magic in the book involves sacrifices of sentient beings, some with very specific requirements as to sex and age - well it's not something I would have come up with but nothing nightmare inducing. It will make SJWs howl, so keeping it where you have to log in and look probably saves them some bad press. And the names given to the rituals ate fantastic, who doesn't want cast "Geometries of the Labyrinthine Spaces" instead of Maze?
2. If my players came to me and asked to me to run it, I would. But contra my thoughts in 1. above, I wouldn't do it around kids. I don't need blow back from the players spouses - or my own. Also, the villains would always be working on a ritual demanding a sacrifice that could be fulfilled with one of the players. Players characters with real skin in the game with a vengeance.
3. The mashup of Cthulhu Mythos background with space aliens and their weaponry is like the love child of Sandy Petersen and Dave Arneson's imagination. Very cool and if I don't run it directly, I'll certainly incorporate pieces in my gaming. You could very easily incorporate as written in RuneQuest, Call of Cthulhu or any other BRP derivative. Other Old School systems would require more work, but nothing too extensive.