I haven't heard of any problems with the N55 and have had 2 cars with the N55 without issue, but that being said, since turbo motors use boost, they don't create vacuum when under boost, so the traditional method of pulling vacuum off the intake doesn't work. To create constant vacuum for things like the brakes, a vacuum pump is needed and the N55 has one (different than the one of the N20/6 though).
The brakes still work if the vacuum pump fails, they just stop being power assisted brakes, but if you pushed hard they would actually work and stop the car, might need 2 feet if you are not that strong, but it wouldn't be a 100% no brakes senario like above.
2008 335i Vert Montego/Saddle/Grey Poplar
2011 Audi S4 Deep See Blue/Black/Stainless
2014 X3 35i M-Sport Space Grey/Chestnut/Fineline Wave