In addition to that, there have been amplification attacks based on ntp. So using an external service increases your attack surface in any future possible breach attempts.
Best security practices dictates to use as less external services as possible.
Same goes for dns and forwarders.
(and the beauty of running a stratum 0 ntp server, over pps, remains with the few who have attempted the task.
Now, I wish datacenters had glass roofs so gps could work on top of racks.. :)