Ultras will work fine, UNTIL they download the latest version. Then they won't and will have the restart issue. My older Ultra is still on the previous version, as are my Roku 3's, and they still work. The new Ultra I got on Black Friday downloaded the latest version and all it does is restart the Roku. I don't know if there is a way to prevent updates, but I have a feeling all Rokus will be broken once they download the latest version.
My Roku Ultra is running 9.0 -- has been running that for weeks -- and it works great. It's not broken like you said it would be. I wonder what I'm doing wrong.
Now new issue with Roksbox Cloud - it doesn't load at all - it gets past the splash screen, I see a quick blip and then my guess is it is trying to reach a webserver and just hangs with a blank screen (it used to do this a long time ago to fetch themes and then time out after 15 seconds with a cached/default theme if it couldn't reach home) - it stays black for at least five minutes - so basically the channel is unusable.
While I didn't care for the update dropping Poster layout, only grid, it DID work on my Roku. now it doesn't work at all.