Yes its true, there are some issues with workouts like weightlifting and so on. It's better to have a chest strap in these cases.
But also the normal workouts have some issues if your watch isn't fitting right. In my case for example there is a special position on my wrist to wear the watch to get the right heart-rate, otherwise I get results like 52,60, 70 BMP instead of the right 140 to 180 BMP.