We have seen some Support Assist issues and best suggestion is to download and install each update manually rather than use the Support Assist. Access the Dell downloads (link below) using your service tag number and it will only show downloads applicable to your specific machine. [URL]
I am running the latest version [3.4.5.366].  It states that there is one urgent and three recommended driver updates but when I click on "Install" it hangs with a status of "Validating" for all four drivers.  This hang lasts at least 10 minutes -- that's when I eventually shutdown SupportAssist.  Before I shut it down I looked at it using Windows 10 Task Manager.  It was using a fair amount of CPU but never did use the network. I have a three-year old  XPS 7760 AIO, running Windows 10.  
SupportAssist hangs while updating drivers.