For a complementary mirror to work properly, the curves of the compensating diodes must match the transconductance curves of the transistors. In discrete circuits, this is not easy to do because of diode and transistor tolerances; so you have to settle for less than perfect mirror action. But diode bias really shines when it comes to linear integrated circuits. Since the compensating diodes and emitter diodes are on the same chip, they have almost identical characteristics; this means almost perfect mirror action over a large temperature range.