The UN and its bodies suffer from Schrödinger’s legitimacy.
In that its legitimacy depends on whether the country in question that talks about it got its way or not.
If the country in question managed to veto (or get the veto holders to veto) a resolution then the UN will be touted as legitimate.
If not, or a resolution they wanted to get adopted fails, then it’s illegitimate, biased and flawed.
Same for its bodies like the ICC, if it goes after a country’s enemy then the ICC is to be respected and it’s legitimate, if the ICC however goes after the country in question then it’s illegitimate.
“Country in question” in this is any given country that happens to be making statements about the UN.
Surprisingly it’s almost never brought up that the UN is made up of individual countries and is, aside from the veto process and fixed seats in some of its bodies, an institution that’s utilizes a democratic process in its decision making.