Honestly it would be quite hard for me to distinguish between 4 and 8, depending on how distracted I am. And also it doesn't distinguish between constant pain and pain that flares up.

Anything above 7 is excluded if it's not constant pain. You might have an episode that would register as a 10, but can't be logged as anything larger than 7 because it's not "all the time".

I'd say it's imperfect, because it is too vague and too restrictive at the same time.