I live in a rural part of Arizona. If I use a high sensitivity SDR receiver connected to an outside, elevated antenna, I can receive about a dozen different transmissions in the ISM band, centered on 443.900 MHz.
Some of them are coming from my own appliances, like a remote reading weather station, a remote thermometer in my wife's green house, etc. The rest come from ??? I have only about five neighbors that are within a radius of 500m, so the other signals are likely coming from them. I can only imagine how many different signals there would be if I lived in a dense urban area.
Being a ham radio operator, I have equipment that transmits in the 420 to 450MHz ham band. I have noticed that when I transmit, it completely kills the ability of any of the ISM devices to receive their respective transmissions. This is due to my legal 100W transmissions at, say, 440MHz, completely blocking the simplistic 434 MHz super-regenerative receivers (exactly one tuned circuit between the antenna and the detector) for the duration that I transmit.
Also, the damn super-regen receivers in the ISM gadgets radiate RFI hash back out their antenna for about +-2Mhz centered around 443.9. Hams have learned to avoid trying to do any weak-signal work close to that frequency. I can imagine that in an urban environment, the ISM band would provide useful communication only very short distances, based on the quantity of such devices that step on each other, and unrelated transmissions coming from Amateur and Commercial Spectrum users.