You are right, of course: if you use new proposals that have not yet been standardised, you are at risk of them breaking in the future. (I've been bitten by this at a previous employer that insisted on using Polymer as well. Don't get me started on that.)
When you stick to standardised features with multiple implementations, though, I think in the vast majority of cases you would've been fine. (This is also one reason why I was against using Polymer.)
(Obviously this does not cover all Electron features, which is why I don't consider browsers to be a replacement for Electron, at this point. I'm merely stating that it's very much possible to target browsers without having to concern yourself much with their rapid pace of development.)
You are right, of course: if you use new proposals that have not yet been standardised, you are at risk of them breaking in the future. (I've been bitten by this at a previous employer that insisted on using Polymer as well. Don't get me started on that.)
When you stick to standardised features with multiple implementations, though, I think in the vast majority of cases you would've been fine. (This is also one reason why I was against using Polymer.)
(Obviously this does not cover all Electron features, which is why I don't consider browsers to be a replacement for Electron, at this point. I'm merely stating that it's very much possible to target browsers without having to concern yourself much with their rapid pace of development.)