The only issue I have had with car rentals in Mexico is with the insurance. They insist of my purchase of "optional liability". If I say that if it is optional, I don't want it. (Keep in mind that in Mexico, the liability insurance in on the car, not the driver. The policy paperwork is in the glovebox.) So when I try to decline it, they say it is required. And at up to $40US/day, it is more than the actual rental.