It's based on HardenedJS.org

The sandbox itself is tight, there's a bug bounty even.

The same technology is behind metamask snaps - plugins in a browser extension.

And Moddable has their own implementation

The biggest problem is endowing too powerful capabilities.

We've got ambitious plans for isolating DOM, but that already failed once before.

so to answer the actual question. if something expects too much browser/dom access to work, it won't?