It's not very reliable at all. In fact, Windows Server Backup (WSB) is a backwards step from NTBackup which was included in previous versions of Windows.
I would not trust the package at all, except for very simple single file server backups. Since you are running Hyper-V, I would definitely consider a more robust package such as Backup Exec, which has support for remote agents in order to back-up your Virtual Machines and any network PCs.
WSB does not support network based backups, and backups to a network share are also *very* limited in functionality. WSB also has no native tape drive support and cannot back-up to any form of tape (even with drivers), so attempting to back-up to tape with WSB is a no-go.
I would suggest either upgrading Backup Exec to 12.5, or considering another backup solution; many people have been recommending 'Backup Assist' recently. I have not used this product myself, however.
-Matt