If you use CONSISTENT=Y and the volume of updates is large, the rollback segment will be large. In addition, the export of each table will be slower because the rollback segment must be scanned for uncommitted transactions.
I don't understand why the rollback seg should be large since if CONSISTENT=Y this should block changes taking place on the relevant nested/partitioned table until that table has been exported. Or are these changes put on hold/queued and then applied immediately after the export of the relevant object. Perhaps someone could clarify.