I feel that they slightly overestimate for me. Although, I'm never quite sure if the prediction is for perfect conditions on a flat route.
Another thing to keep in mind is that watches uses track further distance then the actual course. My fastest official (parkrun) 5k time is around 22:10. Garmin logged the 5k time as 21:xx as it had tracked around 5.1km. This was closer to my Garmin prediction of 21:45
I used to go as far as possible every time i ran, it wasn't until I started reading more about running that I started to slow things down. I started enjoying it more and had the motivation to several times a week, instead of just once.