https://appletoolbox.com/2018/08/apple-watch-walkie-talkie-not-working-how-to-fix/ discusses range, but more importantly the fact that if users are sharing the same Apple ID functionality is doubful:
"If you share an Apple ID, Walkie-Talkie will not work with anyone you share an Apple ID with. Everyone must have their own unique Apple ID to use Walkie-Talkie.
Many people share an Apple ID with their husbands, wives, children, or friends so they can download purchases, share photos, and other services.
This sharing Apple IDs worked well in previous iOS versions, but folks report problems with iOS 12 combining iMessages, FaceTime calls, and Walkie-Talkie not working when Apple IDs are shared between 2 or more people.
Because Walkie Talkie uses FaceTime Audio, you need separate Apple IDs.
FWIW, I have found that statement to be accurate. My wife and I both have watches, share the same Apple ID, and have found Walkie Talkie to be useless.