There is a related consideration. The parts must be fixtured dead-on. Otherwise, you'll be producing high-speed scrap. So, Porter said, dowel crowders and air-checking devices can be necessary in order to assure that the parts are seated right and located properly. Which can add some complexity and mass.
Variations can introduce all manner of part-tracking concerns.
That's not been our experience. The objective is to keep removing metal for as much of the cycle time as possible. So whereas for a transfer line five or six spindles may need to be designed and built, for a high-speed machining module it may take only one or two to complete the same amount of operations. However, given the amount of work performed on the part in a given setup, the given fixture design may be more demanding, as it is configured to minimize interference for the multiple cutting operations. In some instances, one spindle can be machining while the other is undergoing a tool change.