Truth of the matter is i burnt myself once by using a lot of Rex' old add-ons back in the day and ever since then i stay away from 3rd party things, even though there's great stuff being done.
Yeah these addons were great while they lasted. The only reason why I'm not that reluctant to touch 3rd party plugins is the simple fact that in most cases I'm probably able to fix an issue myself if a plugin breaks, at least if it's a relatively simple fix. But I think it demonstrates Ashleys points quite well. 3rd party support can just vanish from one day to the other for any reason, then something breaks and you're kinda screwed. Recent examples: ProUI and LiquidFun, and those were even paid addons, not some random free addon from the exchange.
But there's also the point that there's a lot of things people want that Ashley is reluctant to implement for one or the other reason. So what option is there other than a 3rd party addon?
Prime example is UI stuff. Highly requested for years at this point. Best we got was the HTML plugin. Mind you, that plugin is great for all kinds of purposes and I use it quite some, but not for making a good UI in a game due to the nature of DOM elements being almost entirely detached from the games rendering and always draw on top no matter what (+ some extra annoyances) Yes, implementing it on a canvas level has a rats tail, but it's an important rat regardless.
Just something like flexbox/scrollbox implementation for a start would already be a huge win, but alas.
is this because the official plugins use undocumented features or is it because their code is not publicly available in a plugin-like package?
The latter. You can access the code with the inspector if you know where to look and unminify it, but that's not exactly useful for anything other than looking at it.