With my experience, installing CU's or SP's via WSUS is a slow and painful process where it happens one instance at a time and generally needs a reboot between installs.
I find it MUCH faster to install the CU's and SP's manually on my systems as then you can install them on ALL instances in one go. Mind you, you still need to reboot between CU's and SP's (unless you are 100% certain that a reboot isn't required, then there are flags you can use to skip the reboot check, but I would advise against using them unless you are certain it won't cause problems).
If you install CU's or SP's from the associated EXE, I find it is a lot easier as you just click on the checkboxes to say which instances you want updated. Makes it a TON easier. Plus, if you have any sort of failover set up, you can do all of the pre-install steps without having the update cause a failover in the middle and screw things up.
My opinion, the ONLY use cases for WSUS to push out SQL Server updates would be if I had only 1 instance on the machine and I had no failover in place.
Others on here MAY have nicer experience and tricks to make it work, but I do all of my SQL updates manually. Another advantage to this is I have the ability to control the updates. If they are pushed by WSUS and there is a bug in an update and I want to wait, I need to open a support case with IT and HOPE they disable that one update prior to it getting pushed out. If I do it manually, then I know I am the one who is going to handle the update and I can run it through the test systems first and make sure nothing breaks due to the update.