Are wearables and sleep trackers useful or do they make things worse?

Both, depending on the person. They are reasonably good at tracking duration, steps and heart rate trends, and mediocre at sleep staging, which is the number people care most about. Their real value is spotting trends over weeks rather than judging any single night. The risk is genuine: anxiety about sleep scores can itself worsen sleep, a pattern common enough to have been given a name. If you find yourself checking the score before deciding how you feel, that is the signal to stop. Used as a trend tool and ignored daily, they are useful.