UWP apps use different APIs to interact with windows than Win32. So at least in theory, the OS can actively manage them similar to Android or iOS apps.

I've noticed that Zen 2 on desktop can easily suffer from re-occurring spikes in temperature, caused by background Win32 running on a 10-30 second timer, doing small amount of work that causes the cpu to Boost to it's maximum clock, then drop back down. I could see that mitigated a lot of that with drivers or CPU code that is much slower to ramp up in response to load, but it would be better still if the OS wasn't running any Apps that behaved like that, especially while User isn't actively interacting.

shared resources and the ability for the OS to dictate background activity on a per-(UWP)app basis.

probably more. i'm not that uwp-hep.