I can't answer directly, but having an integral LTC can increase the risk of losing the entire unit in the event of a LTC failure. LTC failures are fairly high on the components of a transformer that can fail. There are some designs that have a Non LTC transformer connected to external voltage regulators. These regulators can be bypassed for maintenance and typically 3 single phase units are utilized. In the event of regulator maintenance or failure, the xfmr can still operate with the units bypassed. The regulators can be removed, repaired or replaced with much less effort than trying to work on an in situ LTC.