What you're missing is that it's not about compute, it's about fun. Pi's are fun, because Pi cluster's have _real_ hardware redundancy and real risk of hardware

What you're missing is that it's not about compute, it's about fun. Pi's are fun, because Pi cluster's have _real_ hardware redundancy and real risk of hardware faults. Clusters of VM's are boring (what's the point of having a cluster then?), and NUCs are too expensive to buy three or five of. Clusters of real servers are even more expensive, and living room incompatible. Energy efficiency does not matter: First, a Pi costs $5 per year in electricity. For the price of one NUC, you can buy the entire Pi cluster _and_ run it for years. Second, this is home use, not enterprise use. Don't assume the nodes are actually constantly utilized. The numbers are a lot less unfavorable for the Pi when you look at realistic workloads, by which I mean idle (~4W/Pi vs ~11W/NUC). I object to calling it "running clusters for no reason". Hands-on experience, fun and motivation are perfectly valid reasons. And this is extremely cheap as far as specialized hardware goes. Please compare with professional training approaches (run 19" servers in lab, or rent EC2 instances), other hobbies (e.g. horseback riding), or professions (e.g. aviation).