If it is not directly made by Fender, you are gambling
There are such things as “Partscasters” which is a guitar body from somewhere, a neck from somewhere, and they are put together by an individual or guitar shop. Another scenario is a completely counterfeit guitar where it may say Fender but it’s a knockoff.
Fender has a plant in Ensenada, Baja California, Mexico. The American made (actually made in Corona, California) is their top of the line guitar (their words, not mine).
So, What’s the Difference?
The main difference is the quality of the components and the quality control standards adhered to at the factory putting the guitar together.
The CNC machines that are used to make the parts to very exacting standards are the same whether in Mexico or in the US. The difference comes with the fit and finish of the components + the expertise of the luthiers (fancy word for guitar builder/technician) doing the final assembly and setup.
Components
The pickups, tuning machines, bridge assembly, nut, and fret wire on an American Strat will be of higher quality than on a Mexican Strat. The wood, either alder or ash, used for the body of the guitar will have been cureddryed to the proper specs. A 2-piece body will be book-matched and the seem will be almost invisible, where the 3-pice body’s two seems will show a little grain mismatch. The paint (actually it’s a polyurethane finish) will be glass smooth and the final assembly will be done with care. All of this adds to the cost, of course.
A Mexican made Stratocaster is a great guitar, especially for the money. The standard, for around $600 new, is a good guitar for a beginner. With a proper setup (things like truss rod adjustment are best done by someone that knows what they are doing) it will play well and sound fine. In my experience, the craftsmanship coming out of the Ensenada plant is quite good. You can get a better neck, better pickups, better bridge (etc, etc) but if you just want a good guitar that you can play right out of the gig bag, the standard is a good choice.
One Caveat
If you are in the market for a Strat and are considering either the Mexican or American, be sure to play the exact guitar you are buying. Wood is a living material and two of the same guitar can feel and sound different. An alder bodied Strat will be a little brighter and weigh a little less than an ash body. A maple neck will be brighter sounding than a rosewood neck.
Final Word
If cost is the main driver and you are a beginner, the Mexican Strat is the best choice. If you are a more experienced player, the extra money for the American is, at the very least, buying you peace of mind knowing that the components are better and the assembly will be top notch.