Earlier quoted context omitted.
It supports backing up to a file that you can't specify. This is unhelpful in many use cases. Example: I am running out of space on my phone. I look, and see Signal is taking 4gb of space. But I would like to preserve a lot of that media. So I buy a mini-SD card and install it in my phone. Yay! Now to turn on Signal chat backups! Oh, but the backups are hardcoded to a location on your primary storage that you can't c…
Even a cynic like me wouldn't go so far as to assume bad intent here. Signal is open source so the file format should be readable and likely this is just an oversight, I guess it's not a highly requested feature. Have you filed an issue?
Backing up to the same device on which are running out of space is an extremely obvious problem, to which the solution is to just ask the user where they want the backup stored instead of deciding for them.