3rd party add-ons such as scenery and aircraft work just fine so long as you purchased them through the Marketplace. There’s no way to install directly from the creator as you have no access to the Community folder. I have the PMDG 738 and the A2A Commanche.
Add-ons that rely upon software running outside of the sim such as BeyondATC and Vatsim do not work.
But these are all limitations that Xbox users already deal with, so there’s no real change there.
As for peripherals, I’ve documented my experience here: Commands duplicated across multiple controllers
It’s not optimal, but for the level of improvement I’m seeing in performance and the visuals, I’m willing to deal with it and holding out hope that Asobo either eventually fixes the controller problem or Nvidia adds native support for more controllers.
As for the stream deck, you can’t run any plugins like AAO or SPAD.next. You can only use it natively for macros and such. I don’t see that ever changing, but who knows?