Older Sonoff Zigbee devices could be a little flaky, but the current stuff is pretty good.
However, I will repeat the concern raised by others… just using the ZBT-1 coordinator is not going to be sufficient. You need a significant number of router devices for a robust and reliable mesh. Having a weak mesh will cause your sensors to drop out randomly, often requiring your to manually re-pair them; and they may also use up their batteries much more quickly than expected.
There are a number of introductory articles about Zigbee available in the Community Cookbook Index.