Reposted here from Facebook for reference:

Not sure how many out there are Fitbiters but I believe this applies equally to all wrist/LED-based HR monitors (e.g. Apple Watch, Garmin devices, and other trackers). My own anecdotal testing corroborates this study as does DC Rainmaker’s review of the Garmin 235, i.e. they are good at low intensity but get less accurate as intensity increases.

NB: The study referenced was commissioned by the plaintiffs in a class action lawsuit against Fitbit, so bias is a strong possibility. The descriptive language could be more neutral but it’s hard to deny the data.

NYT Article
Just How Accurate Are Fitbits? The Jury Is Out

Study
Validation of the Fitbit® SurgeTM and Charge HRTM Fitness Trackers

DC Rainmaker Review of Garmin 235 (HR Accuracy)
Garmin Forerunner 230 & 235 In-Depth Review