I've been using Mr. Tuffy's since 1995 with no problems. Thousands of miles on the streets of NYC and DC, lots of different tires, and only one flat (a nail through a sidewall). If Mr. Tuffy's caused flats, you'd think I would have gotten at least one.
The ones I have came with tapered edges. I understand that some years ago, they changed the way they make them, and they have a sort of blunt edge now. Maybe that's the problem.