When documents are converted formats features, formatting etc will be lost. If you export everything as CSV you loose all your formulas etc. xlsx and ods are better, but not perfect.

If you could export all your documents in the native format and import them into LibreOffice you'd still lose features, but as LibreOffice improved it's import you'd lose less.

With convert at export data is permanently lost. With convert at import, no data lost permanently lost, it just may not be currently available.

Also if you could backup in native format you could import into a google account if your first account is blocked/lost, instead you have to go through the import/export conversion twice.

Also, with Excel, once you have a computer running Windows / Excel, you can't lose data if a feature is changed / dropped. Just don't upgrade. With Google Docs, you have no choice to upgrade to the latest version. It may not even be a dropped feature, you spreadsheets may accidentally rely on a buggy feature, and when it's fixed your spreadsheets behave differently with no warning.