The upgrade made noticeable improvements in both of my radios. The Pro 106 is an older and completely different scanner. It has no yellow cell holder. DMR reception is good and unless a system is busy tracking is not an issue. Both of my radios also handle simulcast in my situation quite well. It is better to not charge AA batteries in any radio. An external charger that charges each cell individually gives maximum cell life and performance.
That's odd, the owners manual for the Pro-106 says they came with both, the yellow and black battery holders as has been the standard for several of the GRE designs sold by RadioShack. I have the GRE made version, the PSR500 and it came with both battery holders also.
Possibly misplaced yours if you are using an external charger. And yes, I fully agree about charging outside of the scanner!
I've always done that and also use per cell chargers like the good Maha model or similar.
As far as a Pro-106 charging circuit problem, I don't think there was a specific problem specific to the Pro-106. If anything, it's probably the same problem that can happen with charging batteries inside any scanner due to the way individual cells cannot be charged or monitored correctly. That can lead to meltdown of your radio if a single cell shorts. But, it can happen to GRE designs just as easily as it can happen to Uniden designs.
I did read the post about someone not being able to charge in the scanner (a Pro-106) even with the proper yellow holder but I'd say that was an isolated incident with that users particular radio and not any widespread Pro-106 only "charging problem".
As popular as the 106 was back in it's day, a charging problem would have been posted all over the place.
Always good advice though about trying to steer users away from charging regular Ni-Cd or NiMH cells in groups inside their radios!
I have a bin of yellow holders here. On some of my older models I used to isolate the charge voltage supply contact inside the battery compartment so I wouldn't accidentally charge in the radio in the event I only had yellow holders to use.