Okay, so this is going to be a rather lengthy explanation in my words, so bear with me.
I won't spoil much, but I will say this. The last moon is on the Darker Side, after kicking the fluffy cotton tails of the Broodals on the Dark Side post game. It. Is. HELL. More Hell than just the Dark Side. Now, technically, it is meant to be the last moon anyone would get, so of course it would probably the hardest moon to get in most eyes. But, still, that doesn't make it any less unfair! I had no choice but to get the last moon with Assist Mode activated and I never came back to that game ever since. Being forced to use Assist Mode to get the last moon and finally 100% Super Mario Odyssey really humiliated me that much.