Many IoT vendors at the last IoT conference in Amsterdam (27-28 June 2018) prefer ZigBee as the connectivity protocol to RF or WiFi due to very low power and advanced functionalities.
However, there are 2 main concerns:
1) it brings proprietaries to the market (must be connected to gateway from the same vendor);
2) the devices are still expensive compared to other connectivity such as RF (nearly 3-5x)
However, a universal ZigBee gateway project is available out there. Check it out: https://github.com/Koenkk/zigbee2mqtt. I used it to connect to my ZigBee sensors at my home.