My reviews
(spoilers are blurred in the dropdown, so you can open everything without fear!)
Disclaimer: It has been a while since I used SR so it could be that I’m misremembering things or can’t be too specific in my reviews!
Easier
The Jam Maker
I dropped it pretty quickly because I have a lot of experience with (mild spoiler, happens in one of the first 5 episodes I think) blind people, and it seemed to portray blindness and living with it very differently from what people I know who are blind have told me, and that annoyed me a bit. But maybe that changed later.
Additionally, it made me too hungry 
Kiki Mimi Radio
My favourite! Somehow mysterious and exciting. I wish there were more stories like this and that it was longer, no other story seemed to grab me as much as this one 
Hole in the Wall
Very cute! I really liked this one. Although (this will become a recurring theme) I didn’t like how we needed an detailed account on (mild spoiler, happens in the first few episodes) how a few of the mouses died.
Sakura and Suzuki
This was the first one I tried and it was such a drag for me. I dropped it about 2/3 of the way through in favour of Kiki Mimi Radio, which I am very happy about.
Intermediate
Kona's Big Adventure
I really liked it! Although it has some very rough descriptions of animal abuse, which I didn’t really want in a graded reader. I prefer mine to be more fluffy or, if they are gruesome, specifically in the thriller/horror genre (like The Neighbour or Koibito).
The River Sanzu
This one I remember the least about. I remember thinking there was too much car vocabulary? I almost fell asleep while reading the first few episodes. Felt very tiring. The ending was nice though.
The Neighbor
I really enjoyed this one. I wish it would have leaned more towards the creepy aspect and didn’t get resolved so quickly.
Koibito
Well, I read that this was the best story in SR, so I guess I was a little too excited for it. I was a bit disappointed that it had a lot of weak points and wasn’t as exciting as I hoped it to be. I think if I’d gone in with lower or even normal expectations I would have enjoyed it more. But all in all it was one of the better stories for sure and a real page turner for a graded reader.
I was really annoyed at the ending though. 
Harder
I dropped them all, so here are some quick impressions
I started Secret, Fujiki Consulting Services and Kona’s Big Adventure II. I dropped Kona’s Big Adventure II because it was too sad to me since (big spoiler) Mocha died. I don’t remember much about Fujiki, but I also dropped it because it got sad (I can’t even remember in what way (except that it was about cats again?)), just that I was a bit disappointed that I couldn’t find a light-hearted story in the vein of Kiki Mimi Radio, and I want a bit more basic stories in my graded readers and don’t want them to hit too close to home or get too depressing. But that’s just me!
I guess it didn’t help that during the year when I used Satori Reader we had 2 heartbreaking cat deaths in the family, so the stories that revolved around sad cat stories were a bit difficult for me to read.
I had nothing against Secret, but my subscription was about to expire and I just thought it was time to move on, as I didn’t enjoy the harder stories that much anyway.