I'm actually of two minds about this supplement, especially when compared to the other two offerings by the same group (Malady Codex I and II). Don't get me wrong, I wholeheartedly recommend it if you are a fan of the Codexes, just be prepared to be disappointed by the quality of some of the writing.
Most of the supplement and the suggested system are quite good, and it gives you a lot of tools and ideas for putting together your own disease. Things are generally well explained and the book is well laid out and put together.
My problem is with the Symptoms tables and the apparent mismatch between them and the rest of the Workshop and the Codexes. It just doesn't feel like the symptom tables were all that well thought out, which is terrible because they're one of the most important parts of the book! The disease building rules are cool, but the end result is only as good as the symptoms you put together.
The first issue is that a lot of the magical symptoms just feel flat and uninspiring, like they were mundane effects that just got the minimal touch-up to make it sound magical (and sometimes not even that). To use an example, one of the magical symptoms is literally "your sense of smell is altered". Another one is "You are unable to eat". There's also "Nightmares. You don't get the benefits of a long rest". If it was just one or two entries I could forgive it, but when it keeps happening it just screams filler. And that brings me to my next point.
My biggest gripe is with the lack of copy editing. A non-trivial portion of the symptom tables just feel thrown together as "filler", or what you would get if you told a group "write 10 symptoms down" and then just combined the lists. The magical symptoms are already largely uninspired, and the lack of proofing just makes it 10x worse.
It might seem like I'm nit-picking, but this lack of copy editing keeps happening throughout the tables:
- Some entries feel like duplicates with slightly different flavoring. A few actually are duplicates. For example, there are two symptoms that mess with a child's growth development that cause the child to be smaller than normal ("small" size for one, 50% species average height for the other). These could have just been a single entry with "the child is significantly smaller than normal, possibly reducing them by one size category".
- The entries are all written with inconsistent voice. Some entries are "you/your" while others are "the creature/it/the child".
- The grammar and syntax are problematic, and I'm not always sure if there was an attempt at humor or just bad english - "Your smell makes other not like you. Sexually". No I did not make a typo there. It is verbatim.
- Tone is all over the place. Some entries are formal, others sound like a post-it note - "You suffer from the hunger effect (exhaustion, check phb)".
Again, each peccadillo is excusable, but on the whole it just seems hastily put together, especially compared to the rest of the supplement and the Codexes. The Codexes are fantastic. The diseases are all different, they're well thought out, they're thematic, and they feel real and scary. The system described in the workshop is cool. I can see how you would use it as a starting point for crafting the diseases in the Codexes. But it's completely let down by the symptom tables. It's like a cool magic system with cookie-cutter spells. Yes, I can work with this. Yes the system is cool. But I gotta put in so much effort to make it work that I'm frustrated and disappointed.
Should you buy this supplement? Yes. The good outweighs the bad. The good parts are fantastic, and it's well worth using the rules if you want to add diseases to your game and flesh out the world. Just be aware that you're buying a system of rules, not a complete workshop. You will need to make your own symptom tables, and that's a damn shame.