Like any piece of software, SUS Server does have some limitations that you should be aware of:
SUS only distributes critical patches – it will not download any driver updates.
SUS only delivers patches for the Windows 2000, XP and 2003 operating systems. It will never download patches for Windows 9x and Windows NT 4
SUS will not deliver patches for Office, ISA, SQL, and Exchange.
SUS will not deliver patches for Non-Microsoft operating systems.
With SUS you cannot roll out patches from the clients, you can only Un-approve, which will restrict future installations, but it will not un-install those from the clients.
SUS lacks reporting features, its tough to know the patches installed at the clients.
With SUS you cannot target the clients; it’s automatically targeted to all those computers configured with AU for local SUS.
Reboots can affect logged in users, there is no way to say NO if a patch requires a reboot.
AU Client will check in SUS Server for Approved patches at a random time, 17-22 hours and it’s not possible to increase/decrease this time.