Heat tape is very safe when connected, regulated and used properly. In a rack system I personally like to use one T-stat per strip of heat tape. Doesn't matter If that strip of flex-watt is heating one tub or six. Some keepers will run multiple strips of heat tape on one T-stat, and It can be done as long as the wattages are the same, but I prefer not to do that. Totally a personal decision.
I place my T-stat probe directly on the flex-watt, right in the middle of the rack. Usually, I run my heat tape at about 102 degrees to achieve a 90 hot spot in the tub (I use paper substrate so your mileage may vary).
Back in 2010, I did have a severe T-stat malfunction due to an electrical storm. My T-stat allowed the heat tape to run unchecked for several hours (I was out of town on business at the time) until the the tubs melting onto the flex-watt overloaded everything and the T-stat shut off power to the heat tape. The result was 7 dead snakes and a ruined rack, but thankfully, no apartment fire. That was a one in a million occurrence, and the only time the manufacturer of that particular model of T-stat has ever seen it happen.