How accurate is the built-in pulse sensor on NordicTrack treadmills?

why is the Polar H10 so much better than the cheaper Polar heart rate straps?

H10 uses better electrodes for heart rate measurements. It also includes dual bluetooth/ant+ wireless connectivity so you can sync to two devices at the same time. It also has internal memory to record your heart rate measurements without wearing your smart watch. The strap is also made from nicer material too. The H9 models have the same accuracy to the H10 but its just more feature focused instead.
 
had a treadmill that measured my heart rate at 240 bpm during a warm up while walking. It started to flash emergency stop on the console screen. Scared to death for my treadmill. Grip sensors on cheap treadmills arent very reliable.
 
Apple Watches optical sensors will show your target heart rate as your cadence if you use them to track your running heart rate. Its a known issue and one that can be very frustrating to deal with. It will not reflect your actual heart rate but your cadence in steps per minute.