PoE devices needs an isolation transformer in order to be compliant, and it takes quite some real estate. However, so does 110V/220V, but i guess we're so used to "hiding" it in a wall-wart.

Something like [0] would basically cover your PoE adapter needs for this, right? Remember that these SBC's also don't directly cover using 120V AC mains power as an option; they rely on a wall wart for that.

The trade-off to the gain in mechanical flexibility of having a very small size is that you have to put the big (in miniature electronics terms) inductors elsewhere.

[0]https://www.amazon.com/TP-Link-Gigabit-Ethernet-Splitter-TL-...

there is a PoE-Hat addon available, at least.

edit: just realized that you meant other projects.