The short version
Nam Tu Liem is a western growth district of stadium land, towers, older villages and unfinished plots. It suits residents who want newer apartments and west side access, especially if work is nearby. It is less convincing as a central Hanoi base because the district is spread out and its roads dominate daily movement.
The area around My Dinh has the most recognisable urban concentration, while the older villages offer cheaper local routines and less polished streets. Choose the block carefully. A tower beside shops is a different life from a tower surrounded by empty land. Families should drive the school route at drop off time, and everyone should ask what is being built next door.
Nam Tu Liem is practical when your life is already west. I would live here for space and a short local commute, not for atmosphere or spontaneous central evenings.