It sounds like you're setting the destinationFileURL.

In that case, we're not updating downloadedLength like we do with data tasks (no destination file URL), however you can provide a download progress block and receive the current status as it updates.

For totalBytesExpectedToWrite, all network clients are at the mercy of the server sending the Content-Length header, which I believe Google Drive does not send when responding to an HTTP GET request. To show a useful progress indicator, in the past I've needed to get the file size from elsewhere (e.g. making a HEAD request before starting the fetch).

With destinationFileURL set, the completion handler's NSData will be intentionally nil. All data is written directly to the file as it's received and not retained in memory. This is useful for large files, files not intended to be used immediately, etc. So the fetcher does not re-load the bytes into memory on completion, avoiding file I/O and memory allocation that are potentially unneeded.

That's pretty much what I guessed, though I don't understand why setting downloadedLength should differ between data and destination tasks, it's useful in both cases?

downloadedLength not being updated is likely just an oversight. No clear reason to me why it's not being updated.