The only real issues on a 2015 Mac would be DRM related with Safari, and native apps such as Prime Video, but using FireFox, or Chrome eliminate those. If you're not doing anything with DRM content, I don't see you having an issue at this time. DRM is an issue with the Apple native stuff because the older graphics drivers are no longer natively bundled with the newer versions of mcOS. You should be able to watch iTunes Apple TV content without issues, as long as you stream it, and don't try to download it. If you do download it, download SD instead of HD, and you shouldn't have DRM related issues there. Everything else just works for me, and the DRM issues for me personally are more of a frustration, of not being able to fix the issue rather than needing it. I use dedicated streamers / TV, or iPhone for content because. they support HDR / multi channel audio while the Mac doesn't.
Edit: Since you probably have a SkyLake CPU in a 2015 model, you have access to AVX2, which will eliminate a lot of issues re3lated to that on older macs that use OCLP that don't have AVX2, and having a metal supported card will also eliminate most issues as well, except the DRM related issues mentioned above.