If you don't have any external library (e.g. npm) dependency you're golden. Unfortunately, this means that you now have to write all your code from scratch, which is ok if you're writing a very light website, but it's unsustainable if you do anything non-trivial.
You mean EcmaScript Modules? The situation is quite complicated. Some libraries don't publish ESM at all (React doesn't iirc), and the ones that do often publish CJS and ESM side by side. In that case, you need to read the package.json and decide which file to use, which is not trivial (see Conditional Exports for example: https://nodejs.org/api/packages.html#conditional-exports). In almost any non-trivial case you need to write tooling to make it work, so you might as well use a bundler.
