Zigbee 101: the profile matters.
Yes, Zigbee was originally deployed in sensor nets. I have worked on some that had literally thousands of sensors.
But it’s not the zigbee home automation profile (ZHA) that smartthings uses.
And most of those sensors are not doing realtime “continuous” reporting. They are battery-operated sleepy devices. That’s not how the consumer oriented “real time energy reporting” mains powered devices work For people who want to know if their television is on or their dryer has stopped.
Also, many of those Sensor nets use a “cluster tree“ topology, not mesh. Zigbee can be configured for several different topologies, including star, cluster tree, Peer to peer mesh etc.
For real time energy reporting, we would likely use the “Zigbee smart energy (SE) “ profile which was specifically not included in zigbee 3.0.
All of that said, I stand by the comments without further explanation for Zwave.
As far as what smartthings engineers have to be concerned about, as they’ve told us many times their typical customer has 15 or fewer devices and never uses any custom code. ( which is probably why they don’t even explain what a repeater is in their setup literature.) I’m not worried about those customers in this regard unless they happen to put 2 zwave realtime energy reporting devices in the same room.