What are you basing this on?  The 6.0 to 6.5 upgrade has been relatively painless for everybody.  It's always been the 5.x to 6.0 upgrade that has been the issue and the steps to do that are in the 5.1 to 6.5 path - you can't avoid the migration by skipping 6.0.

In general, the uglier the source environment in 5.1, the uglier the migration, and it hasn't always been the admin's fault (although sometimes it has been).  Some things worked in earlier releases but were never really documented or supported and NetBackup is not unique in this.  The more complex the product is, the uglier upgrades are going to be since there are too many input permutations to even consider testing.  There are lots of environments out there where Symantec just says "we didn't know anybody was even doing *that*" or "we didn't even know you *could* do that".

If the source environment would have been bugfree since it was installed, it would be easier, but it wasn't - all releases that I've worked on, going back to 3.4 had some set of bugs that would leave the system in weird and wonderful states.  That makes the upgrade even harder since they can't just trust that the source system was pristine.  It also doesn't help that sometimes the upgrade processes themselves have bugs.