Most npm modules are written in JavaScript and when it comes to types they are handwritten (if they exist at all). The very nature of this makes TS unsound, as

Most npm modules are written in JavaScript and when it comes to types they are handwritten (if they exist at all). The very nature of this makes TS unsound, as you have to count on the types being written correctly. I've encountered incorrect types many times. Even worse, when the types ship inside the module and are incorrect, it's difficult to correct them. I prefer JS modules to have a separate @types module.