I think the problem with GTD is that it tries to fit absolutely everything into some stereotype and creates a lot of friction.
Capture absolutely everything(all open loops), fit everything into some project, each project must have a next action, each next action must have a context, constantly maintain the system up to date, do weekly reviews, etc. etc. It’s too rigid of a system. And it expects too much from it’s users. And when your tasks are continuously ambiguous and need flexibility, the system starts to break down.
Here's a newsletter from David Allen which I particularly disagree with:
The major complaint about our Getting Things Done methodology is not that it doesn't work or that the principles aren't sound—it's that people don't work the system. I've learned that many times the problem is not lack of motivation or discipline, but instead some rather mundane and practical behaviors that can be easily changed to make things work much better."
I don't agree that the problem is "mundane and practical behaviors that can be easily changed". The problem is that gtd expects too much from it’s users. It sounds good in theory but some concepts like contexts are not all that relevant anymore and take too much work to maintain. Most things in GTD are just common sense though, so it "works" but I think the whole system is not appropriate for most people. Most people who try to implement GTD don’t ever get to the point where they master the system because it's too difficult to upkeep.