I don't have inside data on the scope of this failure, but I bet it is a minuscule percentage that is causing them to eat a lot of perfectly safe units.
I have a very small set of data points for the issue. I had 3 units that were on the recall list. Since they were small extinguishers, I had replaced them a few years ago with larger volume extinguishers. The 3 were sitting in my basement.
So, when the details of the recall was announced, I thought I would test to see if the 3 I had would actually fail as described. I took them down to the woods and tried them. All of them had the pressure gauge still in the green.
The first one wouldn't work at all. You press down the trigger and ... nothing, not even a trickle of powder. That's one fire that would have continued to burn.
The second one worked well. I got a good spray out it that went for a good distance. Most of the powder stuck to the ground and plants, but the cloud that formed from the remainder was interesting - it just sort-of hung there and slowly drifted further into the woods like it was "The Blob".
The third one also didn't work. You could press the trigger over and over, and some powder would fall out with each trigger pull. In other words, that's the second fire that would have continued to burn.
So, as I said, this is a minuscule set of data, but it indicates a 67% failure rate is possible! The bottom line is, if anyone here has one of the extinguishers that is on the recall list, either take them up on the replacement or replace it yourself. I would not trust a single one of them to work when you need it.