You just made the mistake of saying something was hard. All the "that's not hard, you just suck" folks ALWAYS come out of the woodwork for these situations. It's like having a 5-digit UID and claiming to have been around forever. The 4 and 3-digit folks magically appear. :)
For what it's worth, I have cleared 100% on Mario Kart and Galaxy, but it WAS HARD. Fine, I suck, I can accept that. What I had the hardest time with was when Mario Kart's AI started ramping up the cheating. Mario Kart wasn't hard because of difficulty, it was hard because the AI got massive artificial bonuses.
Mario Galaxy, though, was hard because of intrinsic difficulty, but also the controls. There were times when it felt like the control scheme was designed to make specific levels harder than they had to be. Beating the game is a challenge, but getting 100% is downright brutal.
And then my house got broken into, my Wii stolen. *RAGE*