There is a history available which would be helpful in predicting how much of a problem it might be. You have to look seperately at Canada bound Mexican trucks and border zone Mexican trucks.
As you know, these Mexican trucks are allowed to drive across the US to Canada, I assume since 95. DOT has found a few of these Mexican trucks where they are not supposed to be but the numberes are few. I recall these instances to be less than a 100 per year. Hoffa and his supporters in Congress did a "tip of the iceberg" study that confirmed this. Incidentally, there has been no safety problems either in the US or Canada with these trucks. I have seen a few myself on I 30 between Dallas and Little Rock and except for the script on the vehicle, they look just like a US truck.
As for the trucks limited to the border zone, those violations run about 500 per year. I don't know about other border states, but Texas has a program in place, that is above and beyond DOT, to monitor this.
To allow and regulate will likely result in fewer violations.
I know that many fear the Meican trucks as to what impact they might have US truckers. Let me point out that Hoffa has signed off on the Murry-Shelby amendment. Also let me point out that it was the Mexican govt that banned US truckers in the early 80s to protect their truckers. The US banned Mexican truckers only in retaliation.