I just feel that a lot of Islamic and other terrorist types are getting through and setting up as sleepers that will all come out some day. If and when that day comes, it will indeed be a rough ride.
Crossing the Mexican border illegally gives you the highest chance of getting caught. One can safely assume that real terrorists will seek to minimize their chances of getting caught during infiltration.
Landing by whaleboat on a deserted beach is much easier. Crossing from Canada is much easier. (And the Canucks will let you mooch off their welfare system for quite a few years--all you have to do is say "I'm (name not in Canadian terrorist database) and I'm a refugee." Hell, they'll put you on the fast track to Canadian citizenship.)
BTW, the last method makes you legal, and allows you unlimited access to the United States. It's not even a matter of overstaying a tourist visa. It is the safest means of infiltrating the US.
If you want to shut off illegal immigration from Mexico, say so honestly, and be prepared to justify the costs based solely on the benefits of shutting off illegal immigration from Mexico. It will not stop one single terrorist from entering this country. Selling it as an "anti-terrorism" measure will last until thwe terrorists outsmart you...and then you will have one hell of a credibility problem.