PoE is also fairly bulky, requires large connectors, and either requires a wholly isolated PD or what's basically a class 2 DC/DC converter. That's why PoE-powered stuff usually has that big transformer cube in it with a lot of clearance, slotted PCB, 2-4 kV capacitors etc.

In practice PoE will have lower efficiency than mains powered, since it'll usually be at least double conversion, often three converters in series, plus the losses of the thin network wires, and the relatively high idle losses / poor low-load efficiency of the necessarily over-dimensioned PSE.