Throwing axes are fairly practical. At least, they saw a fair amount of use historically. They’re light weight, and easy enough to train on, but still retain enough mass to do considerable damage on impact.
Throwing knives are more of a novelty trick. They lack the mass of an axe, and are a lot more finicky to connect with. You can throw them, but it’s not a great option.
Spin speed is controlled by the thrower. Really, this one is that simple. It’s a skill they need to practice. As I said earlier, knife throwing is trickier. It’s very easy to screw that up and accidentally connect with the hilt, if you don’t know what you’re doing. In contrast, axe throwing is fairly easy. On contact, the mass of the head will do most of the work for you, so even botching a throw so the haft connects first will usually result in a hit (somewhere). Ideally, with an axe, you want the weapon to make one complete rotation in air, and connect head first.