This isn't something new, but it looks like it could be more important -- I'll explain what happened to me below, when forgetting this rule [it was late] meant restoring a backup. And if I hadn't had that backup handy, who knows?... it could have meant reinstalling 10 plus all my software.
In a nutshell, Before shutting down or restarting Windows 10, take a look in Device Mgr. [right click the task bar to start it] to make sure Windows 10 isn't doing anything... if you've got more than a few % CPU usage, network usage, or more than one or two % disk use, either try to figure out what it is, e.g. an anti virus scan, or safest, Wait.
The potential problem is caused by Windows 10 being much more active behind the scenes than earlier Windows versions -- it'll run housekeeping, update, upgrade, & all sorts of other maintenance tasks when *it* wants to, and unless you look at Task Mgr. while this stuff is going on, you often won't have a clue it's happening.
Making it worse, despite claims that it's doing the reverse, Microsoft is becoming less transparent about this stuff -- when you can find info about updates, that info is most often incomplete. And the only way that you know for sure that an update has been released, is when/if you receive it -- it may or may not be reported by other users.
What happens in my experience is that if you shut down, or shutdown to restart Windows 10, and it's in the process of doing Who Knows What, it can continue working away once Windows 10 appears to close. You may or may not have any sort of progress indicator, just the circular I'm Busy thing, & if the progress indicator does show up with a percentage, it may not be for several minutes. And whatever it's doing may or may not work properly once you've shut Windows down, and may get stuck. If you don't see a progress indicator, there's no good way to tell if it's stalled or in a loop.
If/when you decide 10's stuck, the only way out is to power off, which is what Microsoft directs you to do when setup & similar gets stuck -- the catch is that by doing so, once you power back on, Windows may or may not recover. When/if it doesn't, 10 will try to fix things, which can mean rolling back an update, often to try again once Windows 10 starts. Or it might mean repairing or reinstalling Windows, each of which might cause other problems, may not work, may loose some of your files etc.
Much simpler/easier to let 10 do its thing before you try to close Windows, and that way you at least have some indication of what's going on, can stop stuff & services etc.
Here's what happened to me...
There was news of an update to 10, but when I checked repeatedly 24+ hours later, it still wasn't showing up in Windows Update -- updates show up whenever Microsoft wants them to, and it varies. I did however receive an update that had not been reported. So I updated 10, went back into 7, did a new backup of 10, then bounced back to 10 to do a new backup of 7. [It takes a couple of extra minutes that way but backup image archives are smaller.]
NOW the missing update showed up on its own. It was taking forever so I checked out the individual package download -- it was over 1 GB, yet the update allegedly only fixed the optional Windows 10 mpg2 decoder that comes with their ~$15 DVD add-on, so I let it do it's thing while I went off to do something else in the other room. It finished finally, restarted, and I thought I was good to go. Wrong. I went to shut down the PC when I was finished, forgetting to check Task Mgr. [as above it was late & I was tired], and once 10 shuts down I see the notice that Windows is finishing updating files. What(!)?
After ~20 minutes of this I powered down, then started it up -- showing a low resolution screen (?) Windows said that it was going to try to figure out what was wrong, and then attempt repair or restoration. Powered down, fired up the PC, went into 7, restored that fresh backup, then went into 10 to try that again. This time I monitored Task Mgr., seeing that some time after the update completed, Windows 10 started setting up something having to do with networking. Nothing said Anything remotely about networking. But I let it finish before shutting down, & this time things worked the way they're supposed to.