As much as I love my Garmin Fenix 6 and other multi-sports watches, the wrist-based optical heart rate monitoring is poor.
The accuracy will vary a lot depending on placement and the activity. For me, with things like cycling and rowing, accuracy is poor to the point of being useless. Generally, for things like rowing, it just won't lock on to my heart rate, so a 20min 5K row on the Concept2 it won't register more than 120bpm.
Using an external heart rate monitor will greatly improve accuracy in all activities. There are plenty of options out there, and there are a couple of things you need to consider before buying.
Garmin watches can generally use ANT+ or Bluetooth to connect to a strap, most other watch brands lack ANT+ so you will need to use a Bluetooth device.
Zwift will use Bluetooth for most devices such as phones, tablets, and Apple TV. For a PC you can use Bluetooth or ANT+, but you will need an ANT+ dongle. If you don't have Bluetooth or ANT on your PC you can use the companion app to connect to your phone then the PC, but when I have used this in the past I have found it to be unreliable.
Then you need to decide if you want a chest or optical arm strap. The chest is, in theory, more accurate but I have never noticed much difference. Optical arm devices are generally much more convenient to use and more comfortable.
I have pooled these two together because they are very similar, the more expensive Tri model is suitable for open water swimming as well. These are only worth it if you have a Garmin device with run dynamics, and unlike any other heart rate monitor, these provide additional data about your run.
This includes real-time data for cadence, ground contact time and vertical oscillation.
There is a caveat with these, I have had two HRM-Run straps die on me, and swapping batteries does not appear to help. This is quite a common problem, with 12% of the Amazon reviews giving them a 1-star review. For me, I think the HRM-Run is still worth it, and I use it for outdoor runs only now.
In my opinion, the best all-round hear rate monitor. I have had the original Polar OH1 since early 2018 and it is still going strong. It charges via USB so there are no swapping batteries out so fewer gaps for water to get in and damage the device. It is small, comfortable to wear and the new plus model even has a goggle strap clip for swimming.
This chest strap lacks the run dynamics of the previous Gamin HRM, however, it has a claimed battery life of up to 3.5 years and it is both Bluetooth and ANT+ compatible. One feature that is handy for some, is the ability to concurrently transmit Bluetooth and ANT+ signals to different devices. So you could transmit to Zwift and a Garmin Edge/Fenix etc.
I think this is the cheapest branded solution on the market and it is compatible with both Bluetooth and ANT+. With over 3K reviews on amazon and a score of 4.0, it is a popular device and well worth considering if you want a more affordable option.
I have previously recommended some more affordable options from random brands, but the accuracy of them was questionable, and I think the Wahoo chest strap is cheap enough to not worry about no-name devices.
Last update on 2020-09-23 / Affiliate links / Images from Amazon Product Advertising API