If the instructor can't come up with a quantitative standard of what is acceptable and why the 52X fails, then s/he's either willfully ignorant or dishonest. Either way, I would be looking elsewhere.
Perhaps the manufacturer, Mares, told them this since they are dealers? I know from my days as a retailer/dive shop, manufacturers tell us not to sell a certain model to technical divers because they aren't up to the task for various reasons.
I also know of big name technical instructors who specify the exact model of a certain brand of regulators and dive computer that can be used in their courses and nothing else.