I have used a chest mount (copy) a few times and to get stable footage it needs to be really tight, so much so I find it quite uncomfortable and have to unclip it on climbs to breathe.
I use one of these capturePOV clips now (although again mine is a copy). Fitted to a rucksack shoulder strap its unobtrusive and by cinching down the bag chest strap for descents its just as stable as the go-pro chesty for general riding, with the bonus its simple to slacken the chest strap on my rucksack compared to the chesty.
For DH riding, jumps or ‘stunts’ the chesty will be a better option but the capture mount is really good for days out as it keeps the camera handy and gets good footage.
As another option I found you could get some really good footage from ‘the hip’, i.e. wear the go-pro chesty as a belt with the camera low on your hip. Really good perspective, just watch you don’t clip any trees 😉