Great article, you put into words very well what I, too, see as the big problem about "just do it".

Being a female (and not a feminist) I have a slightly different point of view on it. I don't look for challenges or output-orientated and measurable success. Nevertheless, my experiences are quite the same. "Just do it" can be the ultimative kick of motivation, but it also can put a lot of pressure and too high expectations on people.

The reasons for not doing something - at least not at this moment - don't necessarily have to be fears resp. lack of courage.

Working with depressive people e.g. made me understand very clearly that their suffering is increased by "just do it", because they're simply not able to cope with that.

Indecisive people might not yet have reached the point to be sure what they really want to do. Therefore they also get a somewhat uncomfortable feeling being told "just do it". It's not fear that holds them back but their own subconscious as it has not yet really decided to do it, or maybe it doesn't consider it as most important for the time being. So they might say "I want to do this or that", but their subconscious has not yet told them "ready to go", even if their social environment did.

From my experience (watching myself and others closely) it might be a very strong, motivating slogan, but sometimes it might be more wise to do nothing and maybe wait. I completely trust my intuition to provide me with the best advices I can get, and this has lead me to great satisfaction.
Someone else might deal with it in a different way, and that's where it should be at.
People being so different, so unique makes life interesting and that's the real challenge for me...

These are only a few considerations on a very complex topic.