I built a four-node Raspberry Pi cluster this fortnight, and it has taught me nothing I'll ever use at work. I want to be honest about that up front, because the internet is full of Pi cluster posts pretending otherwise.
Four Pi 3 B+ boards, a stackable acrylic frame, a cheap eight-port switch, and a USB power supply with more amps than the box claimed it had. The fiddly part was nothing exotic. It was the SD cards, the cable lengths, and getting all four to PXE-ish into something I could flash again without unstacking the whole tower. I burned an evening just labelling Ethernet leads.
I put k3s on it, because the full Kubernetes distribution and an ARM board with a gigabyte of RAM are not friends. k3s came up quickly and was genuinely pleasant. I deployed exactly one thing to it: a pod that printed the hostname it was running on, so I could watch the scheduler move it about. That is the entire workload. The cluster exists to show me that the cluster exists.
Did I learn the deep operational lessons that a real cluster teaches? No. There's no load, no failure I didn't cause on purpose, no 3am page. What I got instead was the tactile pleasure of physical kit that does a thing, blinking away on the shelf, mine to break. Sometimes that's the whole point, and I'd stopped letting myself have it. Worth the evening.