How secure is LibreOffice when using password protect?

To be sure (a warning):

As I understand it the question is also concerning the password protection offered by Calc for every single sheet and for the document internally. This protection is completely independent of the protection offered by the Save Dialogues - and completely different, too.

While saving with a password provides a rather secure encryption the internal protection does not provide even a low level of encryption. It is only of use as far as protection against inadvertent changes and against openly showing formulae e.g. is sufficient. I don’t understand any details of the files contained in an .ods container. Nonetheless I just again was successful with cutting out the prorection of one sheet (of 2) and of the document from an example .ods in less than 5 minutes. The example was made with the most recent version V 4.3.3. of Calc.

The only rudiment of actual protection: The internal passwords used are encrypted.