HA platform versus Hubitat and Smartthings, reliability and usability?

Be advised that the connection between Home Assistant and Hubitat is local whereas the official one for SmartThings (SmartThings integration) is not and works via SmartThing’s cloud API.

There is a way to establish a local SmartThings connection but it is a custom solution called SmartThings MQTT Bridge.

There is no official Hubitat integration but there is a custom solution that uses Hubitat’s Maker API (i.e. local connection).