I've never played so I can't really say but your reasons are greatly flawed. Yea you can get ahead of your average player in an online match so even if you get hit with a blue shell you can maintain the lead but what about in a tournament against really good player or pro's? If your playing against other really good players and your in first place and get hit with an unblockable shell the chances are everyone else is right behind, pass you the second you get hit and you lose.
You may enjoy the game but the only way to test the balance of a game is to put it in competitive settings and tournaments. Theres a reason why games like Super Smash Bros, Halo, Street Fighter, Counter Strike and Gran Turismo are used in tournaments. They are some of the most balanced and fair games out there. Mario Kart wii however isn't and hence doesn't have any sort of major tournaments.
Again I'm saying this based off what I've read about the game. If there's a way to turn off the blue shells then that of course nullifies my argument.
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