I’ve seen some argue on the point of energy efficiency, which might make some sense. Mobile CPUs have lower power consumption caps and tend to consume less at idle and laptop PSUs tend to be more efficient at low power draw than desktop-style PSUs.

One might ask why not just use a laptop in that situation, but depending on how the device is used (e.g. always docked), the screen and battery could be seen as a waste.

That said, I think I’d opt for a custom build too, probably in something like the Streacom DB4[0] which allows low power systems to operate fanlessly by doubling as a heatsink.

[0]: https://streacom.com/products/db4-fanless-mini-itx-case/