Tm say if an engine failure occurs during take off, any thrust increase beyond the fixed derate limit could result in loss of directional control....
why we can loss control?
There are aircraft types, INCLUDING a 27K 737/BBJ-1, that require a minimum takeoff weight before use of higher thrust settings are allowed.
If fixed derates are used in cases where takeoff weights are light in these aircraft, these fixed derates become limitations. More thrust would not guarantee controllability because of the immense momentum of the available remaining asymmetric thrust and restricted rudder effectiveness.