The higher-MERV filters are thicker so they can fit in more/deeper pleats to compensate for the denser filter media. More surface area = less air resistance for the same media density. Granted, a 5" MERV 16 filter is still going be more restricted than a 1" blue mesh filter that you could watch TV through, but it’s not like you’re totally choking down your system, assuming it was designed for that size filter in the first place. If your return ducts are already inadequately sized and the filter box is similarly too small, then yes adding a high-MERV filter isn’t a good idea, but that’s not a given circumstance. Also, many relatively modern furnaces will ramp up their fan speed a bit under greater air resistance. I think this is just a property of the particular motor design, not any sort of “smart” control. So it will draw a bit more power in order to maintain the same amount of airflow, no problem.