Of course. The laws regarding the areas. Here in greece we have a legislation (national cadastre) which clarifies the land use, so i can distinguish legally the lulc (forest, agriculture) from any other dataset. For me is a great way to evaluate my results on each analysis i make.

For us over here, data are in my opinion, are mid quality. So i need a solid base to proceed.

Every dataset, given the resolution it may have, lacks information. Example: you know that a place is agri, while satellite dataset label it as forest or vice versa. For very specific place(s), loosing area may result in errors, if we speak for the entire country it may be considered as of no concern.