The problem with "Turnbacks" is that most pilots attempt them at an insufficient altitude and end up crashing or landing well short of the runway.
If you do the research you will discover that many "Turnbacks" are unsuccessful because the pilot does not reach the runway.
As a controller another hazard exists because I may be launching another departure (or several) behind you, and if you make a sudden turn back things can get real ugly real quick.
As someone pointed out, with sufficient altitude a turnback is a safe manuver, but again the key is having enough altitude.
Mike
NATCA FWA