Last night, I was picking up an "intermediate". Admittedly, the rate was quite good, but it was not the best (National were a bit cheaper IIRC). The sample vehicle for the category is the (IMHO rather decent) Nissan Tiida.
I was given what they called a "Dodge Attitude", which is essentially a Hyundai Accent, namely the previous-generation compact Hyundai! Complete with 67k km on the clock and no trim apart from a/c (not even front electric windows). This car is the US sample car for the "Economy" group!
Now that's pretty bad, but the worst part is that they manage to have that car as the sample model for both Compact and Intermediate. Therefore you're paying for (in Hertz's definition) Intermediate and are getting (in Hertz's definition) Compact!
When I realised it and pointed it out, and after a bit of an argument (to be fair, the local employees were quite polite, even after I described the car I had been given as "basura Coreana"), I was given a PT Cruiser, which I guess I am still not really happy with (it probably is too flash for Mexico), but is a damn sight better! According to one of the employees, the owner of the Hertz franchise in Mexico is the owner of the Chrysler importers, and that's why the vast majority of their cars are by that highly-regarded carmaker.
So, to sum up, when renting from Hertz in Mexico, either pay for a high category, make sure that your shiny card (I don't have one, as I usually rent with Sixt who have no presence here) will get you an upgrade beyond "Intermediate", or be prepared to argue that the Attitude barely meets the definition of "compact" and that they should give you something better anyway.