Zigbee can be made easier if you first put a little effort into understanding its limitations (e.i. extremely sensitive to interference, + relative poor reception and low power transmissions on a high frequency) so I do suggest you read enough about it to grasp the concept of its mesh networking technology enough to know how to work around its quirks (i.e. relying on having many Zigbee Router devices to extend range and coverage).
It could also be easy by chance for some simply because they just so happen to start by adding loads of mains-powered Zigbee products that act as Zigbee Router devices and coincidently not place any device close to sources of EMF/EMI/RMI interference, and only after that added Zigbee End Devices (line battery-powered producs).
Personally, I do however not believe in that naive approach, and instead think that researching first before buying anything will give me less of a headache later.
Again, I highly recommend you read and follow this before troubleshooting deeper:
also suggest reading these community guides as well to some gain basic Zigbee knowledge: