Re: Roku Ultra 2018... passthrough for Dolby TrueHD / Atmos , DTS-X, DTS-MA, etc?
Negative. No lossless audio. But the web site is telling the truth, just not in detail. It will passthrough lossy DTS and DD via HDMI to an AVR that can decode it. And the Dolby Atmos it is referring to is the Dolby Digital Plus/Atmos offered by Vudu and a few other online sources. . The only player I know that supports lossless audio bitstreaming for under $200 is the Nvidia Shield. That is precisely the reason I bought one. I still use and enjoy my Roku players and TVs, but for my 4K display I have the Shield to play the audio as expected. This includes TrueHD/Atmos and DTS:X from my UHD Blu Ray rips on my media server.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.