A really good source of info on this is DC Rainmakers blog. His comments [1] on the lawsuit are worth reading if you're interested.

In terms of heart rate accuracy, HR straps aren't always accurate either depending on placement, body and ambient temperature, moisture etc. Certain heart conditions can cause the strap (or maybe more accurately the software processing the data) to significantly underestimate heart rate too.

[1] http://www.dcrainmaker.com/2016/01/fitbits-multiple-lawsuits...