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
