The Edge 800 unit will apparently crash after you ride more than about 200 miles at a time. I'm probably off somewhat on the number, but from what I hear it's reproducible and happens every time. I've used an 800 for many years but never done a ride that long in one sitting so it's never affected me.

The Fenix 3 watch used to get stuck in an infinite loop of fail sometimes where a vibration alarm would go off and not stop, the user's only options were to turn the unit off or wait for the battery to deplete. This only happened with the earliest firmware and was corrected before long. But, two years later, the device still isn't the best in its class for GPS reception, and crashes regularly for rowing and cross country skiing. Their Elevate wHRM is a random number generator.

Apparently the Vivoactive HR double counts its users' calories when they track an activity.

Apart from occasional hardware flops, their main problem is software, and most people feel that their greatest problem is testing, especially regression testing. There are bugs in the software, and new features and bug fixes often introduce new bugs or bring old ones back.