Some Roku devices do have voice control buttons on their remotes.. Both the Fire stick and Roku devices have both Netflix and Prime Video. Google assistant works with some Roku devices, but has some limitations if one doesn't have a Roku TV. I'm elderly and have only used Roku, Android TV boxes, and a Chrome cast, so I can't say if they are better than the Fire TV stick. Neither the Roku or Fire TV sticks are that hard to comprehend. If your elderly person can use an ipad, they should have no problem with the basics of a Roku or Fire TV device.
None of our Rokus have a button for voice input. I do know that there's an app that will allow a Google Home device to work with a Roku but I think it's pretty clumsy. You tell it to click down or up or left or right. It's basically just like using the remote but pressing the buttons with your voice.
If having a voice assistant is important then Fire Stick is the way to go.