See this post :
You can often pick up the APs cheaply on FB market, ebay etc - the smart app needs a controller & bridge (which I run on a raspberry pi) The only thing to look out for is that for the original, square, APs you need to run older controller code (Ubiquiti has the older code on their site) Even if it didn’t do ST integration, I’d still be pleased with ubiquiti wifi kit - it’s really good)
The thing about zigbee over wifi is power consumption. Door sensors, motion sensors are nearly always zigbee as they can run for months on a small lithium battery, wifi devices can’t do that.