I think the email digests for subforum subscriptions are difficult to use since they include the entire text of the post.
For example, I'm subscribed to the News subforum with a daily digest and it's difficult to distinguish where the end and beginnings of posts are when one of them is very long (like this one). Gmail also clips the email since it's so long.
I guess for now I'll switch to "individual/immediate email" rather than "daily digest".