Easy things first. The raspberry pi does not have RTC hardware on board. You can buy third-party RTC hardware that Linux supports and get a RTC running.
For safe shutdown, you should really just issue a shutdown. Either from the commandline or from a gui and then wait for the blinking to stop.
Ubuntu is Debian-based, just like raspbian. Apparently it works on raspberry pi too. I haven't tried it personally. Official-looking Ubuntu documentation seems to say that the raspberry pi 3 is not officially supported, but the '2 is. That's odd IMHO.
The raspberry pi is a bit lacking in IO speed for things like disks. So I'm not sure if you can get more performance out of a fast MSATA ssd, than from the SD card, but you can of course try....
Thanks for the response. We are both on the same line of thinking regarding Ubuntu/Raspbian/Debian. I think I wasn't specific enough in my question/problem statement. I am making reference to the element14 device - Desktop computer kit with expansion board that can turn a Raspberry Pi into a real desktop PC
It does have RTC and with the installed software it executes a safe shutdown that issues the Linux shutdown that you reference. I've run Ubuntu Mate on Pi 3 just for fun. Really everything I do on Raspberry Pi is just for fun. So just wondering if anyone has gone down that path yet so I can find out about pitfalls or issues. Either way after I get Raspbian booting on SSD I'll move on and wipe it and try Ubuntu.
Thanks again for your response. Have a Happy New Year!!
So far all the discussions that I have found surrounding booting a Pi from Pi Desktop SSD involve Raspian only. I've just gotten my Pi Desktop case and booting from SD Card and waiting on my mSATA SSD to arrive. I'll start with Raspian on it just because all the docs cover that in detail. I'm just wondering if Ubuntu will work too? I see numerous references online to booting Mate from USB on Pi. I'm also curious about power switch safe shutdown and RTC support as well with this hardware and Ubuntu.