Running

In the good old days when I wasn't coated in extra flesh, my Garmin one used to rub holes in my skin on my ribs.

It did used to do 50/60 odd miles a week though, so it was probably pretty salty and could walk on its own

never got on with chest straps for running. Either they were too loose and fell down, or tight enough that they felt distracting.

The Suunto mention about warming up before seems the best way I've found to get it to work - seems the watch needs to be expecting the right range otherwise it'll happily find another cadence. It's fine most the time, but if I ever wanted to be sure it was okay then I'd take the Polar arm HR strap.

On longer distance stuff (not running so much, but middle/long distance triathlon) then it just got annoying when I was already a few hours into an event, then on run and then the chest strap starts slipping.

I'm sure I'm doing it wrong, plenty of people cope with the Garmins and/or chest straps.
I'm happy with armband now though, as it also works for swimming (which the Fenix also fails at).

... this reminds me, I should put the the 4 chest straps I have up for sale.

mine went haywire a few years back and was less reliable than my wrist one. Haven't missed it to be honest, I rarely bother to look at HR.

Used to get the occasional chafing from it too. I never raced in one either, too constrictive.