I particularly like this comparison on their webpage:
Security
Managed DNS Service:
Built from the ground up to deliver secure and safe DNS service
UltraDNS does not allow access to server source code
BIND:
The SANS Institute reports that BIND is the #1 security vulnerability
for Internet servers
Hackers have access to source code
Anybody want to make bets that if UltraDNS was anywhere near as popular
as BIND, that the source code would be available to hackers? 
Solaris, VMS, Microsoft, Bell System switches - the list goes on. If
it looks interesting enough, you WILL get hacked, and they WILL have
the source code.
Valdis Kletnieks
Operating Systems Analyst
Virginia Tech