Again, if you are talking about the Shimano Nexus hand-lever operated brake, it is not a drum brake - it is a 'roller' brake.

The front roller brake, I have heard, is not a great option, but I have never figured out why.

I have used the roller brake on an older 7sp Nexus hub and it stops me (260 lbs + bike & gear) just fine. Not great modulation, but fine stopping.
Should work well on a cargo bike, but remamber that most of your braking is int he front and you should get something better (like discs or Vs or properly set up cantis) for the front.