Is potentially dangerous to select the take-off power on airliner lower that temperature at the moment on the airport?
No it's not dangerous.  Flex thrust simply exploits all of the runway available, taking advantage of any length that is surplus to the minimum required at max thrust, to allow takeoff with less than maximum thrust to save wear and tear and fuel (wear and tear is a big deal taking off in a sandy/dusty environment and use of flex has a significant effect on engine life).

The fact that you are using "just enough power" for a flex takeoff does not change the technical risks.  The balanced field length requirements (accelerate/stop and accelerate/go safely) are still met, so the extra length available in a non-flex takeoff does not technically speaking, reduce the risks of something going wrong following an engine failure on takeoff or following a reject.