There are lots of reasons why someone might want to go hybrid with Nintex and Power Automate. I've found with more and more clients that they'll use a Nintex workflow pack (a limited number of workflows with 5 forms per workflow) for long running workflows (since Power Automate terminates after a month) or approvals, or for more complicated document generation - or even just for ease of use. I'm very workflow platform agnostic but I do find Nintex easier to teach business users. Power Automate is better for things like adaptive cards, Teams-integrated approvals, all sorts of connectors....
It is basically algebra to figure out which one to go with for each process, but, it's fun algebra!
In any case, the Nintex-to-PowerAutomate transition is not as simple - especially if going from On Premise. ShareGate, for examples, only supports 30 actions for transition from Nintex On Prem to Nintex O365 ... so now translate that over to an entirely different platform - and - well - just be sure to plan ahead!