Thank you for sharing your experience and your conclusions.
In my case I have a T-665 that works very well, in fact I have stopped using my 2 Garmin gps.
I also had the plastic retaining pin on the charging base break. Since then the charging fails.
I also had problems with the SD card being detected in the middle of a route. Since the maps and tracks were downloaded to the SD card, I couldn’t continue. I removed the tray with the SD card several times without success, until it finally worked…