Don't ignore the advantages of buying a used laptop (e.g. a ThinkPad), either! Battery backup against power loss, super power efficient, keyboard and screen for when your networking or SSH config is screwed up...
Not going to put a battery powered device not made for 24/7 into a corner of my flat, to be running 24/7 and be forgotten about. I've seen one too many puffed up batteries in a laptop to risk that. Nope.
When networking isn't working, SBCs have serial consoles for that.
These days power bricks are not external power sources anymore but merely charging devices.
CPU spikes can create power draws that could not be handled by power bricks as the system is designed as a whole, including a working battery to absorb such spikes, allowing for a downsized power brick (cheaper, simpler, smaller).
Gimping the CPU allows the machine to reliably operate in absence of battery, otherwise CPU spikes would result in power loss.
FWIW, ThinkPads support setting custom battery charge thresholds, which lowers the risk in this scenario. It also prolongs the battery life as the battery is not kept charged at 100% for extended periods of time.
Laptops, at least in my experience, don't turn themselves back on after a power cut so you have to pay attention to them. It was a pain for a CCTV server I had.
