'I can understand problems occuring when a phone's location services are a bit unsure and are just reporting position guestimated from what cell towers it can see. You can see that on Google Maps sometimes, when it paints an enormous circle to say "you're somewhere in here, maybe".'
It's exactly ^this^ which is the problem.
w3w is marketed as being a solution to being found an emergency, pinpointing the location to 3m.
So, perhaps folk download the app and try it at home. Mere seconds pass by, and they get a satellite view of their back garden. Wow, they say. Most impressive!
They don't appreciate they're in range of multiple cell sites, multiple nearby WiFi networks, and have a good strong data signal to help with A-GPS and hotspot/cell tower databases. All these helpers mean their phone can get a location, *fast*.
Then weeks later, out in the sticks, perhaps under some tree cover, in the range of only one cell tower, with no nearby WiFi, no/poor data signal, and in a panic not really knowing exactly where they are.... they open the app and rhyme off the first three words they see to the 999 call handler. They don't notice the massive blue circle. They don't appreciate they're relying on GPS acquisition alone which could take many seconds, perhaps even a minute or two, to get an accurate location. But it's too late. They've passed on those three words and they're in the system now.
Things have got slightly better. w3w produced a script for call handlers to follow; but fundamentally their system is still subject to spelling and transcription errors.
Speaking from experience.