Is there a real problem with NAFTA?
What specifically?
Historically, objections to NAFTA and other FTAs come from the unions and enviros. Right wing opposition is relatively new.
It boils down to protections for the investors versus protection for labor and environment. Investor protections are found mainly in Chapter 11 but also in other Chapters. Mexico took the US into arbitration over the Mexican trucks pursuant to Chapter 20.
The other objection is arbitration. Disputes are settled by an arbitration panel and these decisions trump or supercede national regulations. The courts in the US, Mexico, and Canada have to honor the NAFTA arbitration panel's decisions.
Free Trade agreements are actually Investor-State Trade Agreements and everytime an arbitration hands down a decision on a dispute, that decision establishes a precedent, which is known as Investor State Law. Each and every arbitration panel decision adds to the body of laws governing trade.
For more info on this do a Google search of Investor State Law or Investor State Trade Agreement.
Eventually, over time, as more and more precedents are set by the arbitration panels, a new set or body of laws/regulation are established that supercede the laws/regulations of the individual nations.
There is a leftwing conspiracy theory that the GOP is using these trade agreements as a means to roll back the New Deal. And to make it worse, many of these lefties think that the Clintons and Obama are in cahoots with the GOP. So when Hillary said she was opposed to TPP they all said she was lying and is a Corporatist Suck-Up. They want to see the speeches she gave to Wall Street.
Everybody agrees that there has to be investor protections in these trade agreements. Otherwise, a govt could impose a tax masquerading as a labor or environmental regulation and the investors have to be protected from that. The question then becomes one of degree. To what degree are the investor protections detrimental to protections for labor and environment. In the case of NAFTA, the labor/environmental protections that Clinton negotiated in NAALC & NAAEC turned out to be toothless. Since they weren't actually found within the NAFTA agreement, the arbitration panel gave them no weight.
Everybody agrees that disputes have to be settled by an arbitration panel because the dispute has to be settled quickly and these types of cases can take years to be resolved in the courts. But what happens when a ringer is appointed to the arbitration panel. GW Bush swore up and down that the US lost the Canadian Soft Woods dispute because of a ringer.