Is it possible to use all UWP apps on Windows 10 IoT.
I want to say Yes
All UWP (Universal Windows Platform) applications are designed to run on all Windows 10 API compatible paltforms, including IoT.
But the fact you are asking about this, is scaring me because you may have a UWP that does not work on the 10IoT for some reason??
There are as of writing 3 different API's versions too.. you may need to use the correct OS with the correct API Level
UWP is made for the Windows 10 API ecosystem. UWP also runs on the XBox, Hololens, Surface hub, and "other" compatible OS's
As long as the compiled application is truly 100% Universal Platform then even graphical output will work on the Pi. Albeit the Pi does not have a HardDirectX driver yet and using soft translations.. which also means no h264 streaming. (May 2017) but neither do many other Linux distros
The short answer is NO.
The following are two reasons that readily come to mind about why a Universal Windows Application may not really be universal.
Completeness of Implementation:
UWP encompasses a set of universal framework which is designed such that applications can be ran on all platforms that support UWA (Universal Windows Apps). There are, however, varying degrees of 'completeness' of implementation of UWP. For example, UWP on Windows 10 IoT Core (the version for Raspberry Pi) does not support the FileOpenPicker and so UWP apps that use it (and other similar dialogs) cannot be expected to work properly.
Likewise, if an existing WinForms or WPF application is converted to UWP using the Desktop Bridge with no further modifications, all the underlying calls remain the same with no calls to the universal ones provided by the UWP. These applications aren't true UWA. They are effectively Windows Desktop applications 'disguised' to appear to be UWA. Such applications will not work on platforms except Windows Desktop.