Just my guess. Simplify transforms the condition a*b*c + d*e*f>0 into the complicated result of Reduce[a*b*c + d*e*f>0] and then gets bogged down. It does not recognize that it fits the pattern Simplify[Sqrt[x^2], x > 0]. Funny that the following does work:
With[{x = Sin[a*b*c + d*e*f ]},
Simplify[Sqrt[x^2], x > 0]]
