From a quick read of the github repo and some post from the team I would say that it works for libraries packed as umd that add an instance to the window and correctly instruct unpkg about what file to serve.
Many libraries will work and also many other will not because a lot of them don’t pack the final version in the required format or don’t declare the files correctly in the package.json.
For example randomstring does not pack the library in the required format, ot assumes that you process the soirce file with a bundler. You can easily check that https://unpkg.com/randomstring resolves to module.exports = require("./lib/randomstring"); which is not going to work as is.
Of course this particular library is meant to be used on nodejs, not on the browser, as it’s clearly stated in its description:
A node.js module for generating random strings