Derating, flex, reduced power or whatever they call it, for t/o has been the cause of many problems. Never liked it or been comfortable with it.
When we all began learning, on light a/c it was always full power that was available, that was used for t/o. Using anything less than available you would be foolish in the extreme and you would fail a check if reduced power was used. Quite rightly so.
The heavies I flew, noise was an issue. Standard noise abatement departures required the aircraft to be as high ASAP, V2 + 10 kts until 1500 agl. Max power available within limitations, was used. Easier to reduce power when convenient, than having to apply extra power, should it suddenly be needed. Normally too late then anyway.
Now we have got away from the basics again. All about money I am aware. "If they think safety is expensive, try having an accident". You only need one bad accident in an outfit and it can bring the whole lot down.