PUA and Open Type features

In an old thread, you mention that “there is a rule you should not use encoded glyphs in OpenType features”. Can you tell me more about that? Or send a link?

I am having the same issue as a lot of people where there are people who cannot access all my glyphs because they aren’t using software that supports OpenType. I see that some type designers are using PUA glyphs as a work-around to that. If I do that, could I still write OT features, such as stylistic sets, with PUA coded glyphs? Or will that just mess up the font?

Continuing the discussion from Assigning a Private Use Area to existing glyph?:

You can do that, but it will break everything that relies on I do not recommend it… characters: copy and pasting, switching between fonts, searching, transferring the text, accessibility, text-to-speech, etc.

Therefore, I do not recommend it. It is a hack to temporarily mend a current problem, and no more than that.