I sort of dislike email discussion in many cases because I feel like I need to cover all points and consider all possible replies. Even for areas you know well, simply expressing your points understandably and concisely can take a lot of time and effort, and as you often don't really know exactly which points are important to the recipient, much of that effort might be wasted.

In a face-to-face discussion you can drill down and explore exactly those areas which need exploring... If it's a subject you know well enough to talk about without a lot of offline research, this is often way easier.