Heat cables that go under gravel are designed to dissapate heat over a larger area. They cover a much larger area than a conventional heater. The reason a heater will work in a sump with filter media surrounding it is water circulation.
Try taking a spare tank with no fish, fill with 70F water, add only a heater set for a good 85F. Wait a while, put your hand in the tank some distance from the heater, than closer to the heater. Major temperature difference due to no circulation. Think about long term, under gravel with no circulation. If you want an undergravel heater, just buy one.