BIP39 is good for secrets that need a known degree of security.
When you can easily change your password (eg most web services), this isn’t quite so important.
But with safe, it’s not easy to change. You set it once and that’s pretty-much the end of it.
So if you realize three years later that you picked a bit of a crappy credential, too bad!
This is the only reason I think bip39 is appropriate for credentials. In this case ‘your data in safe’ is essentially the same as ‘your money in bitcoin’ and safe credentials should be treated as such. Maybe I’m too extreme about the value of data?!