Downsides to using window.dispatchEvent to send messages from content script to page scripts?

When sending data from a content script to a page script, both the Chrome docs and the MDN docs provide examples with window.postMessage(). However, window.dispatchEvent() seems to work just fine too, and allows customizing the event name with a CustomEvent. Are there any security or capability downsides to using dispatchEvent instead of postMessage?

There are none and it's the other way around, window.postMessage is a worse solution:

• postMessage's data can be seen by the website's own scripts, while for CustomEvent you can use a randomly generated id;
• postMessage from an extension can break the website if the data is in an unexpected format;
• postMessage has more overhead because its primary use is for cross-origin communication.

The only reason to use window.postMessage is to communicate with the website or with another document in a cross-origin frame or window.