I know that a formula unit is what ionic compounds have. (i.e.  ionic compounds  being not composed of molecules). And I know that the term empirical formula can apply to molecular formulae or to formula units.

Is Formula Unit usually an empirical formula but not always an empirical formula?

e.g. common examples are ionic compounds NaCl and CaCO₃  but from what I understand there are also examples of ionic compounds whose formula unit is not an empirical formula , i.e. the ratio of atoms in the formula are not in their simplest form , for example Hg₂N₂O₆ https://pubchem.ncbi.nlm.nih.gov/compound/129646128   or K₂S₂O₈ https://en.wikipedia.org/wiki/Potassium_persulfate  Their empirical formulae would be HgNO₃ and KSO₄ respectively.

Is Formula Unit usually an empirical formula but not always an empirical formula?

Yes.

Also Hg₂Cl₂.

That is what I am saying.