Most IT devices’ key components run at 3-5 V nowadays and the voltage needs to be converted down if the PoE voltage is higher. This introduces losses, especially if a linear regulator is used as opposed to a buck converter. On the other hand, one powerful PSU is slightly more efficient than a lot of smaller ones.
I don’t think there is a huge difference. If you run a lot of tiny devices (too small/cheap to use a buck converter) off a significantly higher voltage like 24 V or 48 V and/or the cabling is very long, PoE will be less efficient. If the PoE voltage matches the devices’ adapters’ voltage and the cables are reasonably short (<30 m) with few connectors, PoE may be more efficient.