Technically, your concerns are correct. Short of controlling every single enclosure seperately with its own local heat tape and controller, it is impossible to achieve 'precise' temperature uniformity. What determines the temperature of any given segment along a length of heat tape is the speed at which heat leaves that portion of the strip. Physical conditions are inherently nonuniform in rack designs such as these. Thus, everyone settles for temperatures that hopefully are within an acceptable range on racks like the unit in the link you posted.