I use flow with my company account, my school account and my personal outlook.com account.
It's really hard (I'd say impossible) to log into flow using all those accounts at the same time. When I access flow.microsoft.com, I have to select one account, and the only way to change it is logging off this account.
One very annoying situation with this is: imagine I have previously logged into Flow using my school account. Now I access outlook.com (my personal account), and select flow using the apps menu (that one on the top left corner). What happens is that I can only see the flows I created with my school account. I have to log off and log in again to see my personal flows.
Flow should work as the rest of the Office Online apps, tied to my domain.