How to send monthly newsletter if the site is complex and has a lot of content types?
There are a plethora of newsletter generation modules in various states of support. But if you want a first class newsletter that your audience reads you almost certainly want to take the time to write and layout the newsletter in a tool meant for the job (Mailchimp's interface or a competitor).

Newsletter generators are very appealing and can be effective if you have lots of very targeted segmentation of your list. But most of the time if you are sending one news letter to the entire list they are more overhead than value.

Most of the use cases people look to a generator to support are signs of a poor communications strategies. For example: I want to send a daily newsletter about XYZ and don't have the time to write it carefully every day -- does your audience want get and read that much auto-generated mail? Or Laying out my message is too hard to get it right every time -- use a better email newsletter tool that does the job better.