Wrist worn optical heart rate monitors are actually very accurate, under normal circumstances when worn properly. It's easy to verify accuracy yourself by just counting your own pulse rate with your finger. Try it.

Wrist devices will give inaccurate pulse counts if the band is too loose, skin is dirty/wet, or your arm is moving around quickly. If you need accurate numbers in all circumstances then a chest strap which detects electrical signals is a much better option. I usually wear a Garmin HRM-Tri strap while exercising and it transmits a wireless signal to my fitness tracker.