PDF/X is based on an error in thinking by the engineers. This is obvious by the logic behind the implementation. IN PDF/X assigning a CMYK profile to the document removes profiles embedded in any images. This untagging of individual images can lead to color errors if the PDF/X document Output Intent is different from the original color space of the document images.
A universal CMYK document output intent is also problematic because every offset press has its own color space - ISO standards are just "standards" we use to measure from when profiling a press. When a press is "profiled" they will have there own unique color space based on the inks used and the substrate printed on, combined with the characterization of each press. PDF/X sets one standard based on a Color Profile that will have nothing in common with any real world press.
This discrepancy is the same one that affects the Pantone Matching System. To faithfully reproduce a Pantone Solid Color all the print specifics listed in the Pantone Swatch Book will need to be met exactly - paper type, ink type, and press. This never happens for Pantone or for "standard" CMYK profiles.
Because of these limitations, it is best to color manage each image individually, embed a color profile into each image and export with no conversion and preserve the image profiles, and not use PDF/X. The pre-press department at the printshop will then be able to confidently color manage the file for their press. When we receive PDF/X the first thing we do is remove the Output intent because all it does is add a layer to proper color management and can be a false flag.