I'd suggest daily is a definite no-no. For the most part I agree with j and once a week would be fine. The email doesn't become special otherwise and/or the article in question wont have enough time to gain traction within the community.
Personally I wouldn't use it as I'm on hashnode everyday and I still mainly use it for the Q&A section, on the most recent view. I actually hated the Medium emails everyday/week. But I guess the majority of my upset came from the lack of useful and more importantly accurate information within the articles. This kind of moderation is something I would like to see implemented in Hashnode, however I do recognise that this would be quite the complex task. #dictatorship #jokes. I really don't want hashnode to turn into a vortex of useless or just plain bad information that people end up following because its currently the in thing to do. #sorryForTheMiniRant.
Perhaps guidelines can be created on how to create good, accurate and useful information - similar to how stackoverflow suggest a user creates a question.
As for the digest itself, perhaps something along the lines of only the nodes the user is following and the most helpful for that week - if nothing has been posted then nothing gets sent - or could add a button to invite them to create content for the node if not much is going on within said node. Then also choose the specific nodes you wish to be updated on. Maybe even users. I know a few users push out really good content, but only do it occasionally so that would be good to watch for.
It would be interesting to figure out the balance between promoting new content from new users vs new content from existing users who already have a larger 'following'. Giving both a chance to reach the audience.
tl;dr useful => yes. But a fine balance is required to make sure it works and definitely not more than once a week.
EDIT:
Syed Fazle Rahman
Speaking of which - it would be good if users could select which tabs are their home page. i.e. I'd like to hit / and be on /discussions/recent