I have the same scenario as @GoRoos. I was planning on replacing the switch which is on the other side of the bathroom with the GE 12722 and put a SmartSense Temp/Humidity Sensor on the ceiling just above the shower near the exhaust fan. From my research it seems like the humidity sensor’s polling interval is too long and may not kick on the fan until you are done showering. I have emailed SmartThings support to confirm this. @johnathan398 wrote a Smart App called Smart Humidity Fan which I want to try if polling is fast enough. He even put in an auto-shutoff if you turn on the fan manually:
If polling becomes an issue, another approach is what @scottalex has done with a door sensor on the shower door. I’m thinking a SmartSense Multi Sensor would do. I would like to have the fan kick on when the door is opened. Run for 60 seconds, then check temp. If temp is high keep running and check temp again later at whatever the normal polling interval is. Turn the fan off only after the temperature is detected to be back down to normal.