Xiaomi / Aqara motion sensors and door/window contact sensors can work just fine in a SmartThings ecosystem with some important caveats:
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Xiaomi / Aqara Zigbee devices are only known to currently work with a SmartThings v2 hub or SmartThings Home Connect Pro (with firmware v22.52 according to one user), and may work with a v3 hub, but on any other types of SmartThings hubs (ST Link for NVIDIA Shield TV, SmartThings Home Connect (not the pro version), etc.) users have consistently seen Xiaomi devices dropping their connection.
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Xiaomi / Aqara Zigbee devices are also incompatible with a majority of Zigbee devices that act as repeaters. A Zigbee repeater extends a Zigbee mesh network by allowing end devices connect to it instead of directly to the hub. Most wall/mains-powered Zigbee devices can act as repeaters, with some Zigbee bulbs being a notable exception. If a Xiaomi / Aqara Zigbee device connects via an incompatible repeater device, the connection gets dropped after a few hours. The best repeater-capable device known to work with Xiaomi / Aqara devices is an XBee (as explained here), and there are reports that the new IKEA TRADFRI Zigbee Plug is also compatible.
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If a Xiaomi / Aqara Zigbee device’s connection is dropped for other reasons, such as network interference, or a weak signal due to distance, they will not automatically reconnect as Zigbee devices are supposed to. In that case, they need to be manually re-joined (by pairing but without first removing the device from the hub’s device list.)
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The hardware functionality of some Xiaomi / Aqara button devices is reduced because of the way SmartThings handles certain types of incoming ZigBee messages. For example, the Aqara 2-button Wireless Wall Switch can only be recognized as one button in SmartThings.
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Since all Xiaomi / Aqara Zigbee devices use what’s called a custom Device Handler (also known as DTH or DH), they are not officially supported in the new SmartThings mobile app. For now, they should be used with the SmartThings “Classic” mobile app. However this is true for nearly all devices that require a custom Device Handler, and SmartThings is in the process of developing support for them in the new mobile app.
Personally I have over 30 Xiaomi / Aqara devices and have migrated over from my SmartThings v2 hub to the Hubitat home automation platform, partly because of the limits of Xiaomi / Aqara button functionality in SmartThings. I only use XBee devices as repeaters, and so I don’t have any issues with my Xiaomi / Aqara devices dropping their connection (which was also true when they were connected to my SmartThings v2 hub.