IMHO...
STH = More enterprise hardware focus, sold at 2 cents on the dollar. People have the software stack for it already figured out. Or have a good idea about it. More graybeards. Many originals like RolloZ170's VRM 255W fixes, not found elsewhere.
Wendell's Level1 = Perhaps more concept and software solution focus. Many more beginner type questions. "I finally bought a domain name, what do I do now?" Gave win-raid a home when they needed a new one.
r/datahoarder = Storage nutjobs. Dual controller active/active with multipath ZFS is barely good enough. Because, not in your house, not your data, and will go away at most inconvenient time.
r/homelab = I'm told moderation is a problem. Answer quality suggests more of a 'chatty' crowd.
Hardwareluxx = Ze Germans who all run their home labs on 10 watts idle, because electricity here has gotten so expletive expensive, your next invoice might knock your socks off.