From the MSPU editorial, with an inserted word from me:
We’re heading towards the end of 2016 more than two years after Microsoft first introduced Windows 10 and over 4 years since Microsoft introduced the new Windows apps framework in Windows 8 and Microsoft still hasn’t figured out how to make them work for users who want to be productive...
...Yesterday, Microsoft finally brought a new version of paint to Windows 10. Paint 3DPreview, which had been demonstrated at Microsoft’s Windows 10 event last month, now comes built into Windows 10 – replacing the earlier version of Paint that used to ship. I’m very certain Paint 3D is a universal update to the Windows Paint app that had formerly shipped with Windows (ignoring the fact that Microsoft already had a universal Paint branded app in Fresh Paint), but its replacement highlighted one issue with UWP for me that I could no longer avoid by opening the Win32 version of said app. When using Paint to handle multiple images, I would often copy and paste between multiple instances of Paint. Allowing me to work on multiple pictures and then combine them (I don’t do heavy photo editing so paint is enough most times). With the universal Paint app, you can’t do that as Microsoft forces the app to run in only a single instance – and this is a problem with UWP.
To be exact, it has been a problem ever since Microsoft shipped Windows 10. Consider, Universal Windows apps are the future of Window – so Microsoft intends us to be productive and creative on universal apps on our PCs and tablets, yet none of Microsoft’s Windows 10 Apps [currently] supports running multiple instances of a single app. If you decide to go all in on UWP, you can’t view multiple Word Documents, you can’t run Multiple Powerpoints, nor can you open multiple Excel Workbooks, you’re stuck with single instance versions of all.
You can read the whole article here. As the commenters on the linked story have mentioned, the issue isn't with the UWP app platform itself, it's to do with the simplistic way in which applications like Office (and Paint, mentioned) have been implemented so far.
Now, Microsoft's timescales seem skewed to getting everything ready for next spring, i.e. March 2017 and the Creators Update shipping, so let's hope that the heavyweight UWP applications on the platform get reworked in plenty of time. As it is, desktop users will be sticking to Win32 applications most of the time and Continuum users like me will be getting occasionally... frustrated.