Answer:
Actually there are 5 problems related to NAFTA according to critics.
1) Mexico's farmers were put out of business, with free trade US exported agricultural products to mexico at low price. which in turns put local farmers out of business.
2) US jobs were lost, labour in mexico is cheap, so many companies moved their operations and factories to mexico for cheap labour. Which makes the US labour unemployed.
3) At US wages were suppressed, as many companies moved to Mexico other companies used this treat to pay labour low wages.
4) Mexico environment was highly affected, because of high competition local farmer used more fertilizers, fungicides and pesticides which affected their environment a lot.
5) Free US access to Mexican trucks, this was highly opposed by US local trucking companies as they will lose their business if mexican trucks were given free access to US. So, this point was rejected in 2008.