I don't understand the problem. People might work themselves too hard or might not notice their heart is going beserk? If your heart is working too hard, don't you usually feel weird or even like total crap... how does the Fitbit estimate create danger???
Ye because people aim for a certain heart rate - to get into the fat burning zone; for example. If the Fitbit is saying you're using 60% of your capacity and need to push harder to get into that zone, people will push on, thinking they are just unfit and not working hard enough.
Id be surprised if it caused serious issues but it's a risk, and it's disappointing they are so inaccurate given the cost.
There are dozens of different zone calculators, so why believe only one? Are people sheep who can't be expected to think for selves & instead do what the gizmo tells them to? I can set my zones where I like on my HRM (not that I use zones, anyway). Pah.
Mine massively overestimates my calorie burn, good job I don't take it too seriously...imo it's a bit of fun - certainly not an accurate piece of equipment, I got it as a gift from dh....it's a pedometer and the challenges with friends are fun but that's it.