I think it is much easier for many Europeans, Middle easterners and Asians to cross on the Canadian Border. Up in Maine and Vermont you can't even tell when the border is in many places. We used to cross on a friend's family farm with no problem. That border is even more isolated and longer than the Mexican Border too.
That may be the case, but it may be much harder for them to get into Canada....very rugged coastlines....I would expect it to be much easier to get into Latin America...and work your way up thru Mexico.