Just a comment,

while using nfs datasores does have some other limitations, cbt will only
have the drawback that the initial backup will explode thin provisioned
partitions on the initial backup. So take care to have a destination for
the data that will compress on the fly, we get approximately 1:200.
Incremental backups afterwards will definitively only transfer the changed
blocks. We run it for about 1000 VMs per night, so it´s proven to work.
Looks more like you should have a closer look at your ESX environment and
cbt settings if you are only getting full backups.

Regards,

Markus