I am65 and need to be sure I do not exceed my 85% of my max pulse rate when exercising. A HRM also provides motivation to exercise harder to achieve a sensible rate of exercise. FYI my original HRM was a Polar chest strap, but I found it uncomfortable and battery life was an issue, but it was accurate. I migrated to a Fitbit Blaze, and found it to be wildly inaccurate on many exercises. It also had ongoing failures to sync to both Android and iOS, you have to remove the Blaze from its strap to charge the battery, and the glass is fragile. Not Happy! I returned the Blaze and replaced it with a Garmin Vivosmart HR. Found it to be superior in accuracy to Fitbit though it has an concessional transgression when my wrist gets sweaty, wearing it higher up the wrist restores accuracy. Overall performance of the Garmin is good.
I have had 3 Fitbitās now (took each one back as they started to fall apart within the warranty period). The last two had HRMās. My primary purpose was not to monitor HR but the other stats (steps, floors etc), HR monitoring was a nice to have an of secondary importance to me. I also found it to be less than accurate in measuring HR. For example, I would do High intensity interval training on my exercise bike and record my HR the old fashioned way with two fingers on the carotid artery. The manual way gave me a reading of 160-170 bpm, however the Fitbit would read 125!
I have just purchased an Apple watch and am looking forward to getting some stats out of it (have only been wearing it a few days). Although Iām finding it a bit more challenging navigating the Apple watch and related apps than the Fitbit.
I have a Fitbit Charge HR. It was not obvious to me when I bought it that heart rate cannot be conitinuously displayed. Even if I get a chance to press the button to display my hr during exercise the display is not visible in bright sunlight. I have found the hr monitor accurate when compared to my polar chest strap. In the Michael Mosley Trust Me Iām a Doctor episode when they tested heart rate watches I noticed non of them were wearing them correctly. During exercise they do have to be 2 or 3 finger width above the wrist bone. They were instead all worn like a normal watch. I donāt want a smart watch to help me loose weight. I want to see my heart rate to make sure I donāt over do it which I am prone to do. The fitbit was advertised as helping you train in the right zone. Well that is not possible if there is not permanent display.
Iām 70 and do Parkrun every Saturday. I use a Garmin Vivosmart HR which monitors heart/pulse rate in real time. My body tells me when to slow down, my monitor tells me to push harder when Iāve slowwed enough.
Further to my previous comment, Iād also like to point Apple Watch users to an app called Cardiogram, which is (I think) better than Heartwatch. Both are good for different reasons. You can set Cardiogram to record continually. I havent done that, Iām sure it would eat battery.
You can see from the graph that its checking is frequent. The small blue bars are indicators of my pulse dropping under 60. Sometimes it goes under 50. But, I need it to just keep an eye on me. Prior to being treated for my heart issues, I was getting all kinds of crazy readings. That is, heartrate way too fast for good health. For people like me, Apple watch combined with appropriate apps is a godsend.
Following on from the discussion above, weāve made this video: Fitness trackers with heart rate monitors ā are they accurate?
Iād prefer an always on HRM that in particular records periods of irregularities. We were early fitbit users, however after they died, we replaced functionality with the mobileās functionality - when it is on your body!
Very recently have added a Wahoo HRM chest band coupled to the mobile/bike app. It & other accurate HRMās can monitor heart rate variability (HRV). Have not yet delved to see if that is really useful, however I purchased the Wahoo based on it being accurate enough for the job.
The Kardia device is mentioned above. The electrocardiologist expressed delight when I presented the Kardia results after not managing to get a useful Holtor monitor trace. The one issue I had with the Kardia was dry skin. Really helped to wet, brine, or gel your fingers. Also, although I havenāt had an issue with the 5 androids & an ipad Iāve tried it with, I read it will not work at all with some mobile phones.
An article regarding claims that Fitbit and similar trackers are inaccurate in calculating calories burnt.
For me, I use it to get a rough estimate of my movements and exercise. Thereās a sense of accomplishment when you see that.
Iām not that concerned about it not being very accurate.
Iām usually relaxed during that process 
If my fitness tracker shows a pulse, then I know Iām good to go. 
