I have my 401k in a Vanguard Target Date fund with rock bottom expenses at 7 BP. I understand the 4 underlying funds it uses. I understand the schedule it follows to rebalance.
It's not clear to me how Fidelity chooses the underlying funds for their Freedom Series, and they have a high expense ratio at 75 BP and 3 times the turnover of Vanguard's product. That suggests, to me, some degree of active management. If you are a convinced indexer, you should seek more information before investing.