I never had that sort of problem on 50cc. Happened once or twice in 100cc, but not too bad there either.
Now, granted there are still plenty of 150cc races where you don't get shelled right before the finish line, but with so many powerups and the AI getting a better share of items than you... online multiplayer is decent enough, but in comparison to cheating AI's is far better.
My biggest 100cc disaster was on the Moonview Highway, I got blue-shelled halfway through a lap right as I'm about to pass through the first series of toll-gates. Okay, but then one of the local Bob-omb vehicles blows me up from behind, and after that, before I have a chance to hit the gas pedal I get flattened by a semitruck!
And Rainbow Road . . . seriously, half-pipe ramps on Rainbow Road. That is insane, yes, but I still love it.