Thanks for the insight @aixnr! We still recommend redis as the default storage engine, especially for small forums, as it was what NodeBB was originally built against. MongoDB is supported just as well as Redis, so there is no fear of it ever being deprecated or considered a second-class citizen when it comes to support.
As for those stats you were asking about:
NodeBB
28,600 Posts
3,645 Topics
3,867 Users
139.08MB memory used in Redis