> For machine-readable formats, I'd argue length-before-text is simpler to parse than splitting by separators
Yes.
> even before having to add extra application logic to handle this method being fallible (e.g: if you want to use it to store filenames, you now need a check and pop-up about some OS-valid filenames not being supported).
I don't see why you'd need that. CSV does not have anything like that.
That's a higher-level concern. This ascii-delimited format (like CSV) is supposed to be a stupid row/column format. And also simpler to implement than CSV.