I am thinking about Google Assistant or Alexa integration with Hassio.
Do they need Hassio with “remote access” enabled?
If so, can I avoid setting up remote access by using IFTTT or other free cloud service instead?
depends on the setup.
alexa with emulated hue doesnt need you to do anything on your router.
but if you want custom skill you need to give amazon access in some form.
i have no clue about GA, because i dont own one.
Are you sure about that?
Unless something has changed I thought you would need to forward port 8300 (for alexa) or port 80 ( for google) to your HA internal IP address.
Back when I used Emulated_hue I needed to open that port on my router.
i never opened any ports for it and i use it for 1,5 years now.
i got only 1 port from my network (in my router) opened its 443 to my nginx, which points to my alexa api in appdaemon
really no other port is opened anymore.
hmmm…strange. 
Maybe I opened the port and didn’t need to.
i think so, i have seen a lot of confusion about that.