Rockets: Necessary, Primitive. You can work them out in late bronze age or early iron age if you happen to have the right materials nearby. The Kemono-world folks haven't figured them out because they haven't had to. Sooner or later you get a Warner von Braun or the like, and then shit gets real REAL quick. If you're very lucky, Kemono Warner is lawful neutral and just wants to launch to the moon. Sharing that kind of tech isn't really much more dangerous than sharing the recipe for black powder, and leads to things like global communication and opens the door to space exploration as others have mentioned.
Nuclear Power. Not primitive, and in the wrong hands, really, really dangerous. Every time you open a square on the tech tree, you get both a positive and negative application. Nuclear Power is, when done correctly, really safe. The number of people we've had die from the mishandling of Nuclear Power in the last two centuries is NOTHING compared to the number of people we've had die from mishandling of coal power.
But, being able to build your own reactor and support machinery for it means you're also able to start refining nuclear material that can be used in weapons. That way madness lies. You don't HAVE to build centrifuges capable of refining weaponized uranium, as many modern countries have demonstrated, but you CAN. Astronaut-kun has known the Kemono-folks for a few months at this point. He's going to have to know them a LOT better to find someone he can trust with nuclear power.