I have four Raspberry Pi 4s bolted into a little stacked acrylic frame, each with a PoE hat, all hanging off one switch. It runs k3s. It runs absolutely nothing that I need. It is, by any sensible measure, a waste of a weekend and the better part of a hundred quid.
I would do it again tomorrow.
The honest pitch was "learn Kubernetes networking properly". The reality is that everything I actually learned, I could have learned with three VMs on the box already humming away under the desk, and learned it faster, with more RAM and no thermal throttling. ARM made half the container images a faff. The PoE hats run hot enough to cook the SD cards if you don't add the little fans, and the little fans whine. None of this taught me anything about distributed systems I'll carry into work on Monday.
But there is something about physical nodes. When you kubectl drain a box and watch the actual blinkenlights settle, the abstraction stops being an abstraction. You can pull a network cable and feel the cluster notice. I've debugged enough cloud incidents through a dashboard; doing it with my hands on the hardware reconnected a wire somewhere in my head that years of terraform apply had quietly disconnected.
So no, it taught me nothing useful. It just reminded me why I liked this stuff in the first place, which on a wet January Sunday is plenty.