Last night, I tried to upgrade my 2011 Garmin unit with the 4.14 firmware update, which resulted in a bricked navigation screen.
There are two updates floating around, one for the CTP13 (2013) units (v3.15 I believe) and one for the CTP2012 (2011-2012) units (v4.14).
I used the CTP2012 (v4.14) file, and my unit is stuck on the Garmin splashscreen with "Loader Loading..." text on the top. It will not boot any further.
I've tried a couple of things to try to reflash it, including re-flashing the entire Uconnect system with the 2011 update and for the heck of it even tried the CTP13 upgrade.
Nothing worked, so I've put in a request to Garmin to see if they can help me get the "correct" update file, but I'm not holding my breath since it's a OEM unit and I don't think they like people messing with them in the first place...
Luckily, this is my "spare" experimental radio, so if I can't get it working again it's not a huge problem.
In short, I would suggest that if you have no compelling reason to upgrade your Garmin firmware, you shouldn't do it. There are too many different builds out there to be completely sure that your update is 100% compatible. I think that it's safe to say that there are dozens of unique builds for each model depending on which week it was made.
This does NOT mean that there is any trouble updating your maps, this warning is only for using the "GUPDATE.GCD" file to update your firmware.