[Nevcairiel]

We decomposed the new UNIT_SPELLCAST_* GUID some time ago, this is the layout:

3-[server id]-[instance id]-[zone uid]-[spell id]-[cast UID]

So yes, you need to parse this GUID to get the spell id. The cast UID is unique to every single spell cast.
What the last parameter is on the U_S_* events is not yet known. It looks like flags of some sort, but I was unable to determine their meaning.

The GUID has a bunch of advantages, it allows you to clearly group spell cast events together, like the appropriate start with the success or cancel, or whichever.
The previous method the old events offered was generally not very reliable.

Interestingly the FrameXML UI itself seems to not have been updated to properly use these new events.