Heat mats are exactly that a heating pad or some sort, they are a generic name that would include hot water radiator mats, carbon element and wire elements.

UTH is under tank heater a heater specifically designed to be placed under a aquarium. To my knowledge they are all carbon resistive elements.

Flexwatt is made by calorique it is a carbon resistive panel.

Radiant heat panels (RHP) are (all I believe) a carbon resistive panel in a box designed to be top mounted an over tank heater if you see my point.

Kane pads are made by Kane mfg they are a fibreglass/carbon system they are between a UTH and RHP. A sealed, insulated, carbon element rather than an exposed one.

The max temps of each depend on the wattage, size, type, and density of the element. Ie a piece of 17 inch wide flexwatt is only 12w per sq foot compared to 3 inch which is 40w per sq foot. The surface of the 17 is usually below 88º where the surface in the same room temp (80ºF) is often over 120ºF.

I believe that under normal set up conditions there is no need for any heat source to be over 100ºF. I have a very cool room and all my current heaters will not produce surfaces over 100ºF running maxed out. In the event of a T-stat failure having a pad that produces lower temps means lower risks of serious burns and or fires.

there are a number of UTH that are low density elements (intelliheat, kane, ultratherm, exoterra rainforest (they make a high and low density pads rainforest and desert) there are likely more I just an not aware of them.