The Xioami Devices are not certified for the zigbee home automation profile, which is probably why that particular brand is so often reported as dropping off. Also why the pairing process is weird.
I think any certified Z wave sensor or ZHA certified zigbee sensor Will have fewer problems if the mesh is strong.
There is the issue with zigbee sensors that if you have Osram/Sylvania lightbulbs or some of the other zigbee brands paired Directly to the SmartThings hub, you can run into a problem. These devices identify themselves as repeaters to the hub, but when the network is busy they can get into a kind of buffer overflow condition where they drop some of the messages given to them. This can cause sensors to appear to be unresponsive when in fact their sending messages just fine but the bulbs are dropping them. There’s a comment about this in the official support based document near the end.
https://support.smartthings.com/hc/en-us/articles/214191863-How-to-connect-SYLVANIA-Bulbs
But that doesn’t seem to be the problem with the Xioami If you look in those threads, you’ll see that it’s a common issue with those. 
So stick with certified devices, make sure the mesh is strong, and you should be fine, at least as good as SmartThings gets with regard to reliability, which is a separate issue.