By definition "Flexi Task" action in on premise gives you three things:
Of course there is much more, like the option to send notifications only for a selected set of approvers or not send them at all, etc...
In Nintex for O365 "assign task" actions allows you only to respond for the notification email so that you can approve or reject the task without visiting SharePoint at all. But again with limitations - there is a set collection of possible responses and you cannot define your own like you can in on premise.
Other functions like SMS/ Lync/ Delegation etc... are not present. For some you will find workarounds, some are just impossible.
Moreover in Nintex O365 you have two assign task actions, that are nearly the same with one, major difference:
Summarizing - there are no "Flexi Tasks" in O365 as we all know them from on premises