2 out of 5 rating for Tome of Horrors 5E
I backed the kickstarter for Tome of Horrors quite a while ago. I'd never purchased anything from Frog God Games prior to this, but they seemed to have an acceptable pedigree and I had a really good experience with Kobold Press' Tome of Beasts (lot of tomes being slung around these parts) so I was more than willing to give another third party supplement a shot. Sadly, this is the other side of the coin to the works of Kobold Press, as I feel Frog God really dropped the ball here.
Cons: The artwork is incredibly spotty, with some being of so low quality that I would completely believe it if you told me they were swiped from DeviantArt. While not all of them are quite that bad, only a few really had me marveling at the artwork, which for me personally is a large draw of monster-manual type products. Not much else to say on this note, other than the book isn't the treat for the eyes it should have been.
As mentioned by Maceo, repetition is a frequent issue in the book, with a large swathe of the product being devoted to giant vermin, multiple varieties of orcs with minute changes, and a plethora of zombies and the like. While sometimes this can be nice, as with a large expansion to the number of oozes you can run, overall it's a heavily padded product and unless you're running a beetle campaign there's going to be stuff left unused as a result.
Editing issues abound as well, there are multiple entries with typos or references to abilities or attacks that don't exist. Spacing could be used better as well, as Maceo covered, since the stat blocks are smashed together and details can easily be lost on an initial read.
Most egregious to me personally though is that the designers simply don't seem to understand 5e design concepts. I was reading over the demon section earlier, and noticed that their rage demon had wildly inappropriate stats for its CR, including a 26 CON stat, with no stat less than 14 (int) and an average stat of around 18. Likewise, it has an incorrect and impossibly high perception for its stats. I don't miss the days of ridiculous stat bloat and having every high-tier enemy be amazing at everything. The number one reason I buy a monster manual or supplement is so that I don't have to design every monster myself, and when a product requires frequent fact-checking it has drastically lowered its usability in my eyes.
Pros: The size of the book is pretty good, clocking in at 338 pages and including monsters by type and CR tables, both are welcome. Even with all of the aforementioned issues, there's probably something in there worth running simply due to the volume of creatures.
I briefly touched on it above, but the book also does a good job expanding on certain varieties of foes that often get skimmed over, like oozes, golems, and elementals. I particularly like these inclusions not just because they're frequently left out, but because a lot of those categories can be put into almost any campaign.
Conclusion: While I wouldn't say I feel like the money I spent was wholly a waste, I would definitely caution others from pursuing this project. Its numerous issues are likely not to be solved before printing, and unless you own every monster resource out there and just have to have more then there are better things to spend your hard-earned money on. A shame I guess, but not every kickstarter can be a winner.