I think it's likely you are correct that there are no significant safety problem with the first 100 Mexican trucks. And I concede that, assuming that US inspectors enforce the standards for the trucks and don't stop doing so at some time, the trucks themselves would be OK.
I have a concern with future Mexican drivers that could be brought into the program. What is the maximum number of Mexican drivers could be allowed under the treaty eventually? The fact is that when the police in Mexico stop a driver, the driver pays the mordida and there is no record of the incident. If you don't believe me, ask a Mexican.
If the number of Mexican drivers increases to (e.g.) 5,000 they will have to bring in new drivers. I don't understand how they think they are going to evaluate driver safety when there are no real records. There are many bad driving habits that are very common in Mexico and are currently very rare in the US (although these are likely to become more common as more illegal aliens come in), for example:
Passing on a mountain road when the driver cannot see oncoming traffic (oncoming vehicles which are smaller than the passing vehicles traffic are expected to slow down or get out of the way if they can)
Drunk driving. We have too much of it here, but it's much worse in Mexico.
If that were true then you wouldn't be pro-Obama on kicking out the first 100-trucks.