I've already seen two computers where the "Get Windows 10" screen gets stuck. Basically what happens is when the icon is clicked in the task tray and the continue button is clicked, it says "Starting download..." with the spinning circle and goes no further, even when left sitting on that screen for several hours. If the screen is closed and reopened, it's back at the start inviting the user to upgrade.
It makes me wonder just what percentage of people have run into this same situation. Quite a popular search also on Google based on its auto-completion:
The main workaround that generally works is the Media Creation Tool with its "Upgrade this PC now" option.
Although I've only watched the upgrade take place on a handful of computers (mainly at my workplace), so far I haven't seen any installation fail mid way through the actual upgrade. I remember the pain Windows Vista had with its service packs that kept failing during the installation, especially Service Pack 1, resulting in a lengthy reverting process waiting for it to undo and only for the same thing to happen again, such as after trying various hot fixes and the System Readiness Tool that seldom resolved the issue.