I see a lot of us struggle to get accurate bpm measures, quite important to check whether your VO2 and other fitness parameters are really improving.
I found that the sensor on the Fenix 5 is pretty bad. My main issue is that heart rate goes up easily, but then when you are clearly recovering (for example by just stopping) it doesn't go down.
I bought a Garmin HRM which is no longer working, mainly because it drains the battery after a couple of days. I tried resetting it to no avail. Pretty much useless at this point.
I went for a cheaper chest band from other manufacturer. It used to work fine, but since last month is having connectivity issues. You can see it below:
https://photos.app.goo.gl/LeoDcek1bda2ic9J7
I bought a Polar armband which has very promising reviews a few weeks ago. And it has never worked fine from day zero. The issue is the same, connectivity issues throughout the entire work out.
https://photos.app.goo.gl/sGipa7VeWLbfa4QB7
I reached out to customer service and, after 10 days waiting, I got a totally useless answer, since the person tried to focus on why Garmin HRM is no longer working, but I think the important issue here is the fact that there seems to be connectivity issues when using ANT+ sensors 
Anybody has had a similar issue? Workarounds or solutions?