Since these are adventures, I'm going to try not to give too many spoilers away. After all, there may be folks out there who want to play these adventures.
There are three things I judge RPG adventures these days: Did I enjoy reading it? Do I think it would be fun to run or play? Can they be mined for ideas for homebrew adventures?
Vengeance from Beyond is an adventure about the investigator being haunted by an angry ghost who will drive them to insanity if not stopped. Frankly, it is my least favorite of the three adventures and I think it's the weakest.
That's because it really has the narrowest range of options for the player. There's only one way to deal with the ghost. If the player doesn't figure that one way out, that's it for the investigator. It's a bit like a railroad where you have to find the track.
Frankly, that can be a real problem with Call of Cthulhu adventures in general. Sometimes, they read more like a script than an adventure and this is far from the worst case I've read.
Of Grave Concern has the investigator dealing with a zombie wizard who keeps swapping bodies with them with the long term goal of taking over their body forever.
This is my favorite adventure of the lot. The player has a lot of avenues to explore and ways to go about their investigation. It has much more of a sandbox feel. And, while there's only one way to put the zombie down, it's a lot more reasonable than Vengeance from Beyond.
Robinson Gruesome has the investigator become a castaway on a desert island where they have to resonantly Mythos cultists and the horror they worship. It's actually a very simple adventure with a fairly tight timeline for what all the evil types are doing. However, the investigator has a lot of leeway in what they can do.
But what I really like about this adventure is that it can be mined for more ideas for adventures. While the Cthulhu genre is no stranger to Robinsonades (Dagon by Lovecraft for instance), I haven't seen a lot of adventures that use it. You could build a whole campaign around being a stranded island.
All through out all three adventures, there's advice how to handle only having one player and how to make sure that bad dice rolls don't kill them. After all, poor planning and bad decisions on the part of the player can take care of that just that fine.good advice goes a long way towards making this a good supplement.
Monophobia isn't perfect but it does a good job tackling the idea of running one-on-one Call of Cthulhu in three different ways. It's pretty much a must read for anyone interested in that.