The flip side of straight engines is that a I6 has no really bad harmoinc problems... in other words it is a near perfect setup....
BMW doesn't need all the dampers and counter rotating shafts on its I6 engines as many I4s or V6s do because it is such a good package.
IIRC it has no first or second order vibrations that a V6 has.
It also gets away with only 2 camshafts for a DOHC setup (less rotating mass in the valve train as you have half the cam sprockets)
