assumed temp reduced thrust take offs are safer than taking off at perf limited max weight as assumed temp calculations assume a lower 1/2 rho.
Is it unsafe to take off at max weight?
The statistics for long term engine wear and the chance of engine failure are conclusive. Anyone with any doubt should talk to RR or GE. They will be happy to send you some data.
So yes Flex/De Rated take offs are not only safe but statistically safer than full thrust take offs.