Most people who have issues have their SmartThings Hub right next to their wireless router. You should allow about 5 feet of space around the router or any other devices that use wifi (repeaters, desktops) to reduce interference.
I live in a very crowded 2.4 ghz area and don’t have issues with zigbee (about 25 devices). I did purchase a couple of smart outlets to help boost the mesh.
Good read on channel hopping.