Productivity techniques are like study methods. Once you have a reasonably trustworthy system, you're better off focusing on the actual output. It's the same as an exam candidate who needs to settle on a reasonably reliable study method and then concentrate on the actual exam material. Many people have put a great deal of time and effort into developing study methods. But if a candidate keeps digging into study methods instead of focusing on the studying itself, good exam results are hard to come by.
Methodologies and frameworks come in for a lot of criticism. And through that criticism, we learn which ones are better and which are worse. Those that don't work stop being mentioned and get discarded. They keep evolving, and when you get stuck while applying one, you can post a question and people will share their own experience. On Reddit, a wide range of experiences with GTD and Zettelkasten get posted. It's also a good way to personalize a method by drawing on many people's experiences.
When people try a well-known methodology and fail, it's usually because they use it only superficially. It may also be that they couldn't stand the adjustment period it requires. Or they may not have understood the principle or the effect behind it. As the saying goes, good things take time: good things take time to adapt to. In fact, productivity methodologies often require you to change what you were already doing. In other words, negative transfer makes them take even longer.