I think the rigidity of this system is its biggest drawback, as is the case with most productivity systems. For example, if I have two large projects that I need to get done and I only write down one, the other is going to keep nagging at me in the back of my head while I'm working and distract my focus. If I write both of them down, then I've deviated from the system, which in my experience means that by the end of the week, it'll just devolve into a vanilla to-do list grouped by the 'size' of tasks.<p>That's mainly why I switched to GTD for all of my to-do lists and projects. Because my inbox holds literally every task or to-do item that pops into my head during the day, I never have that distracting "am I forgetting something?" feeling during the day. I can process and sort my inbox whenever I have free time, and the flexibility in deciding which tasks should be made into projects, next actions, etc... means that the system always fits my schedule no matter how unpredictable or crazy it is, instead of me having to fit my schedule into the system.