ZigBee isn’t proprietary, It’s an open standard on top of 802.14.5. MQTT is for command and response dispatch; communication with the devices still takes place over a network like e.g., ZWave, ZigBee, WiFi.

The problem is that 2.4GHz is noisy as fuck, regardless of who’s making the noise or how. ZWave is only better because they use 900MHz.