There's something of a misconception when it comes to Tamriel Rebuilt, primarily that it has been in active development for two decades at the level it is at today. In reality, the entire first decade of its "development" was pretty much just idea guys talking about what they thought would be cool while showcasing their own ignorance of the series (like thinking that mithril would be lorebreaking despite it being an established material type in the prior games) or just being goofy (like adding Godzilla). Without going into the history of the mod, it went through two official TES releases, a drought of content due to lack of interest in the game, and side projects that went nowhere before they reorganized and established the modding team into something reminiscent of what it is today. It has still taken a long time, but it comes with a degree of justification. And they release content to the public all the time so there isn't much to complain about.
As for Beyond Skyrim and Skywind, to my understanding they are just vanity projects that are very private and, at least in the case of the former, embroiled in drama that prevents proper work from being done. It's hard to say what else has been going on honestly, because they aren't very open about their work.
