Very.
Although I don't believe it's being used for greenfield hacks as much now, the world largely still runs on workbooks & apps built in Excel + VBA years and years ago. There are entire supply chains that likely run on this built by some analyst a decade or more ago. It remains by far the largest source of Shadow IT there is, and there isn't enough dev time or appetite to untangle these monstrosities into actual apps.
They aren't sandboxed because that would remove the usefulness. The reason VBA+Excel got its tentacles into everything is precisely because its not sandboxed. Anything the user can access is fair game, including network shares, SQL, and Win32 calls.
> they run in such a sandbox
What makes them interesting is that they can talk with the outside world: API calls, databases, the terminal named after a former Democratic primary candidate...
You need a genuine licensed excel to run the file and prepare returns. Thankfully you can file same returns online on the portal for free so they get a safe pass that way.