1 in 3 (last year) or 1 in 4 (this year) isn't too horrible when you have a few years to apply before they stop issuing TN(/OPT) extensions. That said, you could get crazy unlucky and not get it each time, in which case, that would really suck. You do have other avenues at that point at some companies, like going the L-1 route, but you'd still have to move.