It seems accurate for running. Some people post pics of burning a lot of cals while sick. I always felt like it overestimated calories burned while sick or while I play volleyball. I have a custom activity for volleyball. It's sand volleyball. I forgot to turn the GPS on as I use it for indoor sand vball as well. Here's a screenshot from yesterday:
Calories: 2129
Time: 6 hrs
AVG HR: 117 bpm
https://imgur.com/a/IpgMPim
Oh, and it was about 12k steps and 7mi for the activity. I'm guessing the cals burned is closer to half of 2.1k?
[edit] Hmm, actually it might be right. I did a 16.5 mi jog today in 3 hrs. 140 bpm. 1962 cals. Also, I remembered I walked home last week. That was 4.10 miles / 70 min / 491 cals / 127 bpm. <-- this one is prob the best apples to apples example. It's in line with the vball example. Crazy.