Pulled the trigger on that three times now.
My first cluster was Raspis and ODroids. The two issues I had - not everything runs on Arm (yet) and memory constraints were tough to work with.
My second cluster - 4 Atomic Pis. The (Intel Atom) quad cores were fine but the 2GB of memory was a real problem when I started playing with more interesting workloads.
The current "production" home cluster is 4 old laptops. They're earlier gen i7s and I've dropped 16GB of memory and 1TB spinning disks in each one. I've never been happier with a cluster. I've been free to spin whatever I want up and it just works. Using RKE to setup Kubernetes takes about 5 minutes to build a cluster and I'm using Longhorn for replicated container native storage. I've run experiments for work and my own experience. Whether its Redis / Cassandra cluster or a Kubernetes Operator I wrote, it can always handle it.