Two things actually. First was performance compared to my zwave devices. I had an Ecolink sensor and Iris sensor on the same door, and the Iris (and SmartThings multi 1st gen) would clearly report faster. When triggering automations, or even speaking to Alexa, it was clearly apparent. That’s for both open/close and motion sensors.
Second was form factor. The Iris door/window sensors are much smaller than the Ecolink/monoprice sensors, and way cheaper than Aeon’s little corner device, and the Sensative Strips device. They’re also readily available at Lowe’s. The motion sensors are also outstanding devices, with a tiny size, very lightweight, and easy placement options.
Part of what’s helping my zigbee experience is the elimination of all my GE Link bulbs (ZLL 2700k temp ones), and my tuneable white Osrams. Those have been replaced with GE Daylight bulbs and CREE Daylight zigbee bulbs. The Osrams should be OK now with firmware updates, but my remaining GE’s and the CREE’s haven’t caused me issues at all since they’ve been installed.
What’s also helping my zigbee experience are all my smartplugs (Lowe’s) and Halo smoke/CO devices that are also repeaters.