I have a 2012 comfortmatic. Before i bought it ...it was a delivery vehicle. This means lots of reversing up driveways. Mine works fine when reversing slow, but if you take your foot off the brake, it accelerates and then something jams on making it shudder, forcing you to stop.
Ive spoken to many others who have the same problem. It still works for what i use it for now (camper) ...as i do minimal revesing, but if i needed it for business it would definately require repairing.
So my conclusion is this is a common problem, and build weak-spot. My guess would also be the external mechanism on top of the gearbox.
Probably the main cause for this would be impatiently putting it in forward gear whilst still reversing...which would not damage the gearbox, as it has built in safeguards against damage, but the comfortmatic mechaanism would be getting mixed meassages which would cause a mechanical malfunction, causing the mechanism to mis-align or jam. Its probably not a complicated repair on the mechanism itself,...but like most things in the engine bay, its very hard to access, and this means frustration , and $$$s