Can you prototype any kind of product? Less user-oriented products (e.g. APIs) are harder to prototype, and you probably can’t test every aspect of it, but there must be something you can still test with a prototype.
Low and high-fidelity prototypes, when to use and why.
All the questions and discussion about what prototyping tools is the best reminds me of a similar question among photographers. They focus so much on the right camera/lens that they forget about what’s really important. Don’t go crazy about the tools, tools are secondary, mindset and methodology – primary. Keep this in mind.
Super important to clearly know what you want to learn before you create your prototype. What question are you trying to answer? This will guide your prototyping, the type and fidelity, and even the scope.