I received my Nebula Mars yesterday and while I haven't had much time to play with it, I can report that it natively supports both Airplay from Apple devices and Chromecast from Android devices. No need to plug in an AppleTV or Chromecast into the HDMI port.
It seems to run a version of Android TV and supports loading apps, but the App Store it links to only has YouTube (works well), TedTV (didn't try), HBO Go (seems to be broken), and an old version of Kodi (v.16, aka Jarvis) which I didn't load. I believe you can side load app via the USB port if you can get the proper apk file. I look forward to playing with it more once I get a proper screen. Hopefully, I can figure out how to load Plex and connect it to my Plex server.
Oh, one strange note, I took it out of the box immediately and plugged it into an outlet hoping to charge it up so that I could use it later, but it didn't charge itself until after I had turned it on and played with it for a few minutes. After that, I was able to power it down and plug it back in to charge it. Luckily, it did come charged at about 60%.