Power transformers need to be efficient at line frequency, and they need to be able to pass current without fatigue. It's nice if they are physically robust too, with sturdy construction and wiring and with laminations that don't vibrate and buzz. But basically, they work at a single frequency and they aren't required to do much but transform line voltage to the levels required.

Output and interstage transformers are required to pass a wide frequency range and may be in the presence of reactive components. They are asked to pass a bandwith of three decades and to be flat even if other reactive values in the circuit change. So in my way of thinking, these are really pretty critical components.

Just my 2¢