I'm the Communications Director for a statewide nonprofit in NC that has over 30 local chapters. I think NationBuilder is a great fit for 80-90% of our local chapters, and I've been encouraging them to adopt it, and also building initial sites and backend databases for a few chapters.
The one thing that is consistently a problem is the lack of any household functionality. Its annoying for tracking fundraising, but then at least you can assign everyone at the same address to one fundraiser- very time consuming, but the end result is satisfactory enough.
Mail merges (for postal mailings) are a different animal all together. There is no good way of pulling together folks at the same address and merging their names in a way that makes sense for a mailing. All of our local chapters (which I believe fall into nationbuilder's sweet spot market) rely extensively on mailings.
Really there are three options, noe of them good:
1) Spend staff time with an exported contact list, finding entries that share an address and putting them together before you do a mail merge
2) Track people in Nationbuilder as couples/families- a really bad way of doing things ant not how NB was set up to function, or
3) Use some excel magic or a third party application- and I don't know enough about excel to fix the problem (and I'm not aware of a third party app that owuld help either )