i have never used hass.io
when i needed to choose between those two, i went with openhab because people were saying that it is easier to use.
my personal experience with it...... if you spend 3-4 hours studying.... then you might think of it as "simple". it has a lot of addons, it supports IFTTT out of the box, google home and for sure alexa. what i hate about it is that it sucks on network security (or there is something i am missing) and that it's user interface is crap (but same goes for hass.io).
finally.... yes you can use openhab offline.
the reasons i am using it is because i wanted to use some IFTTT conditions, i wanted to be able to control some d-link smart plugs and some wifi rgb controllers i have in collaboration with my xiaomi products. as for the google assistant or alexa.... personally i couldn't care less for the voice control (but it works for those of you who are interested and you can turn for example a xiaomi smart plug on or off via voice).
at the moment i am waiting for the sony addon for openhab so that i can turn off my tv when i long press my xiaomi wireless switch