Beasts of the Jungle Rot is...it's just swell.
The supplement includes a large, diverse range of dinosaurs with a range of Challenge Ratings. In tandem with Tomb of Annihilation, there should be a wide variety of encounter options to pick from. As a childhood dino-nerd and Michael Crichton fan, I'll give a special nod to the (admittedly Hollywood) venom-spitting Dilophosaur and the wonderfully feathery specimens present here.
There are a total of four entries with Legendary Actions; the Gigantosaurus and the Titanosaurus, and their undead versions. I'm extremely pleased to see 'boss' dinosaurs at midrange CR (Titanosaurus is the lowest at CR 7) that have these legendary actions: they really help demonstrate the immensity of these creatures in an encounter without isolating them to high-CR battles. I'll also note that the undead versions of these two entries have their own zombie-mode legendary actions.
In fact, many of the undead dinosaur variants have fun abilities that make them extra deadly. Whether it's a hungering bite or actual undead dinosaur..."droppings? Droppings?" I was glad to see extra effort and variety making these entries distinct from their living counterparts.
Finally, the book includes a few appendixes that will be fantastic for different player approaches. The inclusion of rules for dino-hunts was exactly the kind of predictive content I hoped to see, and solidifies how this product pairs up with Tomb of Annihilation. I know a few players I've DM'd for who would be interested in utilizing dinosaur hides or selling off their eggs, and making sure to include these terrible lizards in those options will be beneficial.