I think that as long as you’re addressing stable class names, then you’re generally pretty safe with custom CSS/HTML. I use quite a bit of both in my apps, and I’m yet to have any nasty surprises.
As far as the official Glide position on this goes, my reading of that is “if you do crazy stuff with CSS and it breaks your app, then don’t expect us to fix that for you”. And I believe that’s fair enough. Do it at your own risk.
As time progresses, my expectation is that Glide will continue to provide more and more configuration and customisation options, and the need for custom CSS will be reduced. I think this topic is good evidence of that. It’s also not that long ago that we were using custom CSS to create floating buttons. Now it’s a standard feature.
As far as RTL support goes, I have no idea if it’s on the road map, but I do know that many people have asked for it. So I’m sure that the Glide team are aware that there is a demand.